#include <bits/stdc++.h>
using namespace std;
vector<int> to[10005];
int n;
vector<int> dist;
void dfs(int v, int d = 0, int p = -1) {
  dist[v] = d;
  for (int u : to[v]) {
    if (u == p) continue;
    dfs(u, d + 1, v);
  }
}
vector<int> calcDist(int s) {
  dist = vector<int>(n);
  dfs(s);
  return dist;
}
int main() {
  cin >> n;
  int s, t;
  cin >> s >> t;
  --s;
  --t;
  for (int i = 0; i < n - 1; i++) {
    int a, b;
    cin >> a >> b;
    --a;
    --b;
    to[a].push_back(b);
    to[b].push_back(a);
  }
  vector<int> distS = calcDist(s);
  vector<int> distT = calcDist(t);
  int mx = 0;
  for (int i = 0; i < n; i++) {
    if (distS[i] < distT[i]) {
      mx = max(mx, distT[i]);
    }
  }
  int ans = mx - 1;
  cout << ans << endl;
  return 0;
}
