#include <bits/stdc++.h>
#define ii pair<int, int>
#define fi first
#define se second
#define inf 0x3f3f3f3f
using namespace std;
using ll = long long;
const ll mod=1e9+7;
const int nx=1e5+5;
int n, m, a, b, par[nx], dp[nx];
vector<int> adj[nx];
vector<ii> path;
void go(int u)
{
for(int v:adj[u])
if(v!=par[u])
par[v]=u, go(v);
}
void dfs(int p, int u, int s, int t)
{
vector<int> tmp;
tmp.clear();
for(int v:adj[u])
{
if(ii(u, v)==ii(s, t) || ii(u, v)==ii(t, s) || v==p)
continue;
dfs(u, v, s, t);
tmp.emplace_back(dp[v]);
}
dp[u]=0;
sort(tmp.begin(), tmp.end(), greater<int>());
for(int i = 0; i < tmp.size(); i++)
dp[u]=max(dp[u], tmp[i]+i+1);
}
int solve(int s, int t)
{
dfs(0, a, s, t);
dfs(0, b, s, t);
return max(dp[a], dp[b]);
}
int main()
{
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
cin>>n>>a>>b;
for(int i = 1; i < n; i++)
{
int u, v;
cin>>u>>v;
adj[u].emplace_back(v);
adj[v].emplace_back(u);
}
// cin>>m;
// if(m==1) cin>>a;
// else cin>>a>>b;
// if(m==1 || a==b)
// {
// dfs(0, a, 0, 0);
// return cout<<dp[a], 0;
// }
if(a==b)
{
dfs(0, a, 0, 0);
return cout<<dp[a], 0;
}
int cur=b;
go(a);
path.emplace_back(b, par[b]);
while(par[b]!=a)
b=par[b], path.emplace_back(b, par[b]);
b=cur;
int d=0, c=path.size()-1, g;
while(d+1<c)
{
g=(d+c)>>1;
int x=solve(path[g].fi, path[g].se);
int y=solve(path[g+1].fi, path[g+1].se);
if(x>y) d=g;
else if(x==y) return cout<<x, 0;
else c=g;
}
cout<<min(solve(path[d].fi, path[d].se), solve(path[c].fi, path[c].se));
}
