#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=3e5+5;
int n, m, a, b, par[nx], dp[nx];
vector<int> adj[nx], path;
void go(int u)
{
for(int v:adj[u])
if(v!=par[u])
par[v]=u, go(v);
}
int 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;
tmp.emplace_back(dfs(u, v, s, t));
}
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);
return dp[u];
}
int solve(int s)
{
return max(dfs(0, a, s, par[s]), dfs(0, b, s, par[s]));
}
int main()
{
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
// freopen("tdl.inp", "r", stdin);
// freopen("tdl.out", "w", stdout);
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)
return cout<<dfs(0, a, 0, 0), 0;
go(a);
for(int i = b; i != a; i = par[i])
path.emplace_back(i);
int d=0, c=path.size()-1, g, pos=0;
while(d<=c)
{
g=(d+c)/2;
if(dfs(0, a, path[g], par[path[g]])-dfs(0, b, path[g], par[path[g]])>0) pos=g, d=g+1;
else c=g-1;
}
int ans=solve(path[pos]);
if(pos>0) ans=min(ans, solve(path[pos-1]));
if(pos<(int)path.size()-1) ans=min(ans, solve(path[pos+1]));
cout<<ans;
}
