#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;

#define REP(i,n) for(long long i = 0; i < (n); i++)
#define FOR(i, m, n) for(long long i = (m);i < (n); ++i)
#define ALL(obj) (obj).begin(),(obj).end()

template<class T> using V = vector<T>;
template<class T, class U> using P = pair<T, U>;

const ll MOD = (ll)1e9 + 7;
const ll MOD2 = 998244353;
const ll LLINF = (ll)1e18;
const ll INTINF = (ll)1e9;
const long double PI = 3.1415926535897932384626433;

template <class T> void corner(bool flg, T hoge) {if (flg) {cout << hoge << endl; exit(0);}}
template <class T, class U>ostream &operator<<(ostream &o, const map<T, U>&obj) {o << "{"; for (auto &x : obj) o << " {" << x.first << " : " << x.second << "}" << ","; o << " }"; return o;}
template <class T>ostream &operator<<(ostream &o, const set<T>&obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr) o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
template <class T>ostream &operator<<(ostream &o, const vector<T>&obj) {o << "{"; for (int i = 0; i < (int)obj.size(); ++i)o << (i > 0 ? ", " : "") << obj[i]; o << "}"; return o;}
template <class T, class U>ostream &operator<<(ostream &o, const pair<T, U>&obj) {o << "{" << obj.first << ", " << obj.second << "}"; return o;}
template <template <class tmp>  class T, class U> ostream &operator<<(ostream &o, const T<U> &obj) {o << "{"; for (auto itr = obj.begin(); itr != obj.end(); ++itr)o << (itr != obj.begin() ? ", " : "") << *itr; o << "}"; return o;}
void print(void) {cout << endl;}
template <class Head> void print(Head&& head) {cout << head;print();}
template <class Head, class... Tail> void print(Head&& head, Tail&&... tail) {cout << head << " ";print(forward<Tail>(tail)...);}
template <class T> void chmax(T& a, const T b){a=max<T>(a,b);}
template <class T> void chmin(T& a, const T b){a=min<T>(a,b);}
void YN(bool flg) {cout << ((flg) ? "YES" : "NO") << endl;}
void Yn(bool flg) {cout << ((flg) ? "Yes" : "No") << endl;}
void yn(bool flg) {cout << ((flg) ? "yes" : "no") << endl;}

long long Maximum_Rectangle(vector<long long>& ar){
	ar.push_back(0);
	stack<pair<long long,long long>> st;
	long long res = 0;
	for(long long r = 0; r < ar.size(); ++r){
		long long vr = ar[r];
		if(st.empty()){
			st.push({vr,r});
			continue;
		}

		long long vl = st.top().first, l = st.top().second;
		if(vl < vr) st.push({vr,r});
		if(vl < vr || vl == vr)	continue;
		
		while(vl > vr) {
			res = max(res,vl*(r - l));
			st.pop();

			if(st.size() && st.top().first > vr) vl = st.top().first, l = st.top().second;
			else break;
		}
		st.push({vr,l});
	}
	ar.pop_back();
	return res;
}

int main() {

	ll H,W; cin >> H >> W;
	string S1,S2;
	cin >> S1;
	V<V<ll>> flg(H,V<ll>(W,0)),cnt(H,V<ll>(W+1,0));	
	for(int i = 0; i < H - 1; ++i){
		cin >> S2;
		for(int j = 0; j < W - 1; ++j){
			int a = (S1[j] == '#');
			int b = (S1[j + 1] == '#');
			int c = (S2[j] == '#');
			int d = (S2[j + 1] == '#');
			int tmp = a+b+c+d;
			if(tmp == 1 || tmp == 3) flg[i][j] = 1;			
		}
		S1 = S2;
	}

	for(int j = 0; j < W; ++j){
		if(!flg[0][j]) cnt[0][j] = 1;
		for(int i = 1; i < H; ++i){
			if(flg[i][j]) continue;
			cnt[i][j] = cnt[i-1][j] + 1;
		}
	}
	ll ans = max(H,W);
	for(int i = 1; i < H; ++i){
		stack<pair<ll,ll>> st;
		st.push({cnt[i-1][0]+1,0});
		for(ll j = 1; j <= W; ++j){
			
			ll vj = cnt[i-1][j]+1;
			if(st.empty()){
				st.push({vj,j});
				continue;
			}

			ll vk = st.top().first, k = st.top().second;
	
			if(vk < vj) st.push({vj,j});
			if(vk < vj || vk == vj)	continue;

			while(1){
				chmax(ans,vk*(j - k + (j!=W?1:0)));
				st.pop();
				if(st.size() && st.top().first > vj) vk = st.top().first, k = st.top().second;
				else break;
			}
			st.push({vj,k});
		}
	}
	//for(int i = 0; i < H; ++i) print(i,cnt[i]);
	cout << ans << endl;

	return 0;
}

#include<iostream>
#include<set>
#include <bitset>
#include<queue>
#include<vector>
#include<map>
#include<stack>
#include <cstdio>
#include<algorithm>
#include <sstream>
#include<string>
#include<string.h>
#include <cmath>
#include <iomanip>
#include <string>
#include<list>
#include <limits>
#include <numeric>
#include <type_traits>
#define int long long
#define ll long long
#define mod  1000000007
#define inf 1e17
#define rep(i,j,n) for(int i=j;i<n;i++)
#define P pair<int,int>
#define ps push_back
double pi = 3.141592653589793;
using namespace std;
template<typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val){
    fill( (T*)array, (T*)(array+N), val );
}
//ここから始めよう
int p[214514],q[214514],c[214151];
vector<P> G[614514];
int lastid=0;
map<P,int> sid;
int getid(int v,int c){
    P p(v,c);
    if(sid.find(p)!=sid.end())return sid[p];
    return sid[p]=lastid++;
}
void addedge(int p,int q,int c){
    int v=getid(p,c);int u=getid(q,c);
    G[v].ps(P(u,0));
    G[u].ps(P(v,0));
}
signed main(){
    int n,m;cin>>n>>m;
    rep(i,0,m){
        cin>>p[i]>>q[i]>>c[i];
        p[i]--;q[i]--;
        addedge(p[i],q[i],c[i]);
    }
    rep(i,0,m){
        G[getid(p[i],c[i])].ps(P(lastid+p[i],1));
        G[getid(q[i],c[i])].ps(P(lastid+q[i],1));
        G[lastid+p[i]].ps(P(getid(p[i],c[i]),1));
        G[lastid+q[i]].ps(P(getid(q[i],c[i]),1));
    }
    vector<int> d(lastid+n,mod);d[lastid]=0;
    priority_queue<P> que;
    que.push(P(0,lastid));
    while(!que.empty()){
        auto p=que.top();que.pop();
        int u=p.second,dist=-p.first;
        if(dist>d[u])continue;
        for(auto e:G[u]){
            int v=e.first,c=e.second;
            if(d[v]>d[u]+c){
                d[v]=d[u]+c;
                que.push(P(-d[v],v));
            }
        }
    }int ans=d[lastid+n-1];
    if(ans==mod)ans=-1;
    else ans/=2;
    cout<<ans<<endl;return 0;
}