#include<bits/stdc++.h>
#define TASKNAME "codeforce"
#define pb push_back
#define pli pair<int,int>
#define fi first
#define se second
#define fastio ios_base::sync_with_stdio(false); cin.tie(NULL);
using namespace std;
using ll=int;
const ll maxN=3e5+10;
const ll inf=1e18;
const ll mod=1e9+7;
ll dp[maxN];
vector<ll>g[maxN];
ll sz,vc[maxN];
void dfs1(ll u,ll p,ll cam)
{
dp[u]=0;
for(auto v:g[u])
{
if(v!=p&&v!=cam)
{
dfs1(v,u,cam);
}
}
sz=0;
for(auto v:g[u])
{
if(v!=p&&v!=cam)
{
sz++;
vc[sz]=dp[v];
}
}
sort(vc+1,vc+sz+1,greater<ll>());
for(int i=1;i<=sz;i++) dp[u]=max(dp[u],vc[i]+i);
}
ll P[maxN][19],h[maxN];
ll anc(ll u,ll k)
{
for(int i=18;i>=1;i--) if(k>>i&1) u=P[u][i];
return u;
}
void dfs(ll u,ll p)
{
P[u][0]=p;
for(int i=1;i<=18;i++) P[u][i]=P[P[u][i-1]][i-1];
for(int v:g[u])
{
if(v!=p)
{
h[v]=h[u]+1;
dfs(v,u);
}
}
}
ll b,a;
ll cal(ll mid)
{
ll x=anc(b,mid);
ll y=P[x][0];
dfs1(a,a,x);
dfs1(b,b,y);
return max(dp[a],dp[b]);
}
ll n;
void solve()
{
cin >> n >> a >> b;
for(int i=1;i<n;i++)
{
ll u,v;
cin >> u >> v;
g[u].pb(v);
g[v].pb(u);
}
dfs(a,a);
ll dis=h[b]-h[a];
ll low=1,high=dis-1;
while(low<=high)
{
ll mid=low+high>>1;
if(cal(mid)<cal(mid-1)) low=mid+1;
else high=mid-1;
}
cout << cal(high);
}
int main()
{
fastio
//freopen(TASKNAME".INP","r",stdin);
//freopen(TASKNAME".OUT","w",stdout);
solve();
}
