#include <bits/stdc++.h>
#define st first
#define nd second
#define mp make_pair
#define pb push_back
#define orta ((bas+son)>>1)
#define N 1000005
using namespace std;
typedef vector < int > vi;
int n, m, a, b, x, y, bas, son, c[N];
vector < int > g[N];
bool dfs(int node, int par){
if(node == b){
c[++m] = node;
return 1;
}
for(int i = 0; i < g[node].size(); i++)
if(g[node][i] != par)
if(dfs(g[node][i], node)){
c[++m] = node;
return 1;
}
return 0;
}
inline bool git(int a, int b){return !((a == x and b == y) or (b == x and a == y));}
int bul(int node, int par){
vi d;
for(int i = 0; i < g[node].size(); i++)
if(g[node][i] != par and git(node, g[node][i]))
d.pb(bul(g[node][i], node));
sort(d.begin(), d.end());
reverse(d.begin(), d.end());
int mx = 0;
for(int i = 0; i < d.size(); i++)
mx = max(mx, i + 1 + d[i]);
return mx;
}
int main(){
// freopen("in.txt", "r", stdin);
// freopen("out.txt", "w", stdout);
scanf("%d %d %d",&n ,&a ,&b);
for(int i = 1; i < n; i++){
int u, v;
scanf("%d %d",&u ,&v);
g[u].pb(v);
g[v].pb(u);
}
dfs(a, 0);
reverse(c + 1, c + m + 1);
// for(int i = 1; i <= m; i++)cout << c[i] << " ";cout << endl;
bas = 1;
son = m;
while(son - bas > 2){
x = c[orta];
y = c[orta + 1];
int ilk = bul(a, 0);
int iki = bul(b, 0);
if(ilk > iki)
son = orta + 1;
else
bas = orta;
}
// cout << c[bas] << " " << c[son] << endl;
x = c[bas];
y = c[bas + 1];
// cout << bul(a, 0) << " " << bul(b, 0) << endl;
int mn = max(bul(a, 0) , bul(b, 0));
if(son == bas + 1)
printf("%d\n",mn);
else{
x = c[bas + 1];
y = c[son];
printf("%d\n", min(mn, max(bul(a, 0) , bul(b, 0)) ));
}
return 0;
}
