#include <bits/stdc++.h>
using namespace std;
using pii = pair<int, int>;

int main(){
  long long int n, m;
  cin >> n >> m;
  vector<int> d(n, 0);
  vector<vector<int>> E(n);
  for(int i=0;i<m;i++){
    int a, b;
    cin >> a >> b;
    a--;b--;
    E[a].emplace_back(b);
    E[b].emplace_back(a);
  }
  vector<bool> visited(n, false);
  function<void(int)> dfs = [&](int cur){
    visited[cur] = true;
    for(auto&& to:E[cur]){
       if (!visited[to]){
         d[to] = d[cur] + 1;
         dfs(to);
       }
    }
  };
  dfs(0);
  bool is_digraph = true;
  for(int i=0;i<n;i++){
    for(auto&& t:E[i]){
      if(!((d[i]^d[t])&1)){
        is_digraph = false; 
      }
    }
  }
  // cerr << is_digraph << endl;
  // for(auto&& r:d)cerr << r << " ";
  // cerr << endl;
  if(is_digraph){
    long long int even = 0;
    for(auto&& r:d)even += (r % 2 == 0);
    cout << even * (n - even) - m << endl;
  }else{
    cout << n * (n - 1) / 2 - m << endl; 
  }
}

#include <bits/stdc++.h>
#define _GLIBCXX_DEBUG
using namespace std;
using ll = long long;
using vec = vector<ll>;
using vect = vector<double>;
using Graph = vector<vector<ll>>;
#define loop(i, n) for (ll i = 0; i < n; i++)
#define Loop(i, m, n) for (ll i = m; i < n; i++)
#define pool(i, n) for (ll i = n; i >= 0; i--)
#define Pool(i, m, n) for (ll i = n; i >= m; i--)
#define mod 1000000007ll
//#define mod 998244353ll
#define flagcount __builtin_popcount
#define flag(x) (1ll << x)
#define flagadd(bit, x) bit |= flag(x)
#define flagpop(bit, x) bit &= ~flag(x)
#define flagon(bit, i) bit &flag(i)
#define flagoff(bit, i) !(bit & (1ll << i))
#define all(v) v.begin(), v.end()
#define low2way(v, x) lower_bound(all(v), x)
#define high2way(v, x) upper_bound(all(v), x)
#define idx_lower(v, x) (distance(v.begin(), low2way(v, x)))  //配列vでx未満の要素数を返す
#define idx_upper(v, x) (distance(v.begin(), high2way(v, x))) //配列vでx以下の要素数を返す
#define idx_lower2(v, x) (v.size() - idx_lower(v, x))         //配列vでx以上の要素数を返す
#define idx_upper2(v, x) (v.size() - idx_upper(v, x))         //配列vでxより大きい要素の数を返す
#define putout(a) cout << a << endl
#define Gput(a, b) G[a].push_back(b)
#define Sum(v) accumulate(all(v), 0ll)
#define gcd(x, y) __gcd(x, y)
ll ctoi(char c)
{
    if (c >= '0' && c <= '9')
    {
        return c - '0';
    }
    return 0;
}
template <typename T>
T lcm(T x, T y)
{
    T z = gcd(x, y);
    return x * y / z;
}
template <typename T>
bool primejudge(T n)
{
    if (n < 2)
        return false;
    else if (n == 2)
        return true;
    else if (n % 2 == 0)
        return false;
    double sqrtn = sqrt(n);
    for (T i = 3; i < sqrtn + 1; i++)
    {
        if (n % i == 0)
        {
            return false;
        }
        i++;
    }
    return true;
}
//場合によって使い分ける
//const ll dx[4]={1,0,-1,0};
//const ll dy[4]={0,1,0,-1};
const ll dx[8] = {1, 1, 0, -1, -1, -1, 0, 1};
const ll dy[8] = {0, 1, 1, 1, 0, -1, -1, -1};
//2次元配列の宣言
//vector<vector<ll>> field(h, vector<ll>(w));
Graph G;
//始点sからS回以内の移動でたどりつける町を探索
void bfs(vec &dist, ll s)
{
    queue<ll> que;
    dist[s] = 0;
    que.push(s);
    while (!que.empty())
    {
        ll v = que.front();
        que.pop();
        for (auto next_v : G[v])
        {
            if (dist[next_v] != -1)
                continue;
            dist[next_v] = dist[v] + 1;
            que.push(next_v);
        }
    }
}
int main()
{
    cout << fixed << setprecision(30);
    ll N;
    ll x, y;
    cin >> N >> x >> y;
    x--;
    y--;
    G.resize(N);
    loop(i, N - 1)
    {
        ll a, b;
        cin >> a >> b;
        a--;
        b--;
        G[a].push_back(b);
        G[b].push_back(a);
    }
    if (G[x].size() == 1 && G[x][0] == y)
    {
        putout(0);
        return 0;
    }
    vector<ll> distT(N, -1), distA(N, -1);
    bfs(distT, x);
    bfs(distA, y);
    ll ans = 0;
    loop(i, N)
    {
        if (distT[i] < distA[i])
            ans = max(ans, distA[i] - 1);
    }
    putout(ans);
    return 0;
}
