#include<iostream>
#include<algorithm>
#include<vector>
#include<iomanip>
#include<cmath>


using namespace std;

#define rep(i,N) for(int i=0;i<N;i++) //0から
#define REP(i,N) for(int j=i;j<N;j++) //iから

typedef long long ll;




ll binary_search(vector<ll> a, ll n, ll key){
    ll right = n, left = -1;
    ll md = (right + left) / 2;

    while(right - left > 1){
        if(a[md] <= key){
            right = md;
        }else{
            left = md;
        }
        md = (right + left) / 2;
    }
    if(left == -1) return -1; //無い場合
    return right;
}

vector<ll> prime;
void Prime(ll n){ //線形篩,素数列挙
    vector<ll> p(n,0);
    p[0]=1;
    p[1]=1;
    for(ll i=2;i<n;i++){
        if(p[i]==0){
            prime.push_back(i);
            p[i]=i;
        }
        ll k=prime.size();
        for(ll j=0;j<k && i*prime[j]<n && prime[j]<=p[i];j++){
            p[i*prime[j]]=prime[j];
        }
    }
}

#define MOD ((ll)1e+9 + 7)




//cout<<fixed<<setprecision(10);
int main(){
    int n,m;
    cin>>n>>m;
    vector<int> L(m),R(m+1);
    rep(i,m){
        cin>>L[i]>>R[i];
        L[i]--;
    }
    vector<int> cnt(n+1,0);
    rep(i,m){
        cnt[L[i]]++;
        cnt[R[i]]--;
    }
    rep(i,n){
        cnt[i+1]+=cnt[i];
    }
    int ans=0;
    rep(i,n){
        if(cnt[i]==m){
            ans++;
        }
    }
    cout<<ans<<endl;

    return 0;
}


#include <bits/stdc++.h>
using namespace std;
 
#define ll long long 
#define inf 0x3f3f3f3f
#define infll 0x3f3f3f3f3f3f3f3f
#define all(x) (x).begin(),(x).end()
#define pb push_back
#define ii pair<int,int>
#define vii vector<ii>
#define vi vector<int>

#define F_OR(i, a, b, s) for (int i=(a); (s)>0?i<(b):i>(b); i+=(s))
#define F_OR1(e) F_OR(i, 0, e, 1)
#define F_OR2(i, e) F_OR(i, 0, e, 1)
#define F_OR3(i, b, e) F_OR(i, b, e, 1)
#define F_OR4(i, b, e, s) F_OR(i, b, e, s)
#define GET5(a, b, c, d, e, ...) e
#define F_ORC(...) GET5(__VA_ARGS__, F_OR4, F_OR3, F_OR2, F_OR1)
#define FOR(...) F_ORC(__VA_ARGS__)(__VA_ARGS__)
#define EACH(x, a) for (auto& x: a)
 
// debug templates 
#define debug(x)       cerr<< #x <<" : "<< x << endl;
#define debuga(A,N)    cerr<< #A <<" : [";for(int i = 0; i<N;i++) cerr<<A[i]<<" "; cerr<<"]\n";
#define debuga2(A,N,M) cerr<< #A << " : \n"; for(int i=0;i<N;i++){cerr<<"[";for(int j=0;j<M;++j) cerr<<A[i][j]<<" ";cerr<<"]\n";}
#define debugp(p)      cerr<< #p <<" : "<<"("<<(p).first<<","<<(p).second<<")\n";
#define debugv(v)      cerr<< #v <<" : "<<"[";for(int i = 0; i< (v).size(); i++) cerr<<v[i]<<" "; cerr<<"]\n";
#define debugv2(v)     cerr<< #v << " : \n"; for(int i=0;i<v.size();i++){cerr<<"[";for(int j=0;j<(v[0].size());++j) cerr<<v[i][j]<<" ";cerr<<"]\n";}
#define debugs(m)      cerr<< #m <<" : [ "; for(auto itr = m.begin(); itr!=m.end();itr++) cerr<<*itr<<" "; cerr<<"]\n";
#define debugm(m)      cerr<< #m <<" : [ "; for(auto itr = m.begin();itr!=m.end(); itr++) cerr<<"("<<itr->first<<","<<itr->second<<") ";cerr<<"]\n";
 

void solve(){
	int n,m; cin >> n >> m;
	int big_l = 0, small_r = inf;
	FOR(m){
		int l,r; cin >> l >> r;
		big_l = max(l,big_l);
		small_r = min(small_r, r);
	}
	cout << max(small_r - big_l + 1,0);
}

 
int main(){
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	
	solve();
	return 0;
}
 
