#include <bits/stdc++.h>
using namespace std;
#define int long long
const int inf = 1e18;
const int mod = 998244353;
void solve(){
int n, a, b;
cin >> n >> a >> b;
a -= 1;
b -= 1;
vector<vector<pair<int,int>>>g(n);
vector<pair<int,int>>vs;
for(int i = 0;i<n-1;i++){
int x, y;
cin >> x >> y;
x -= 1;
y -= 1;
g[x].push_back({y, i});
g[y].push_back({x, i});
vs.push_back({x, y});
}
vector<int>p(n), u, op(n);
auto dfs = [&](auto self, int x, int pr) -> void {
for(auto [to, ind] : g[x]){
if(to == pr)continue;
p[to] = x; op[to] = ind;
self(self, to, x);
}
};
dfs(dfs, a, -1);
int uio = b;
while(uio != a){
u.push_back(op[uio]);
uio = p[uio];
}
//~ cout << ";;;;\n";
//~ for(int i = 0;i<n;i++){
//~ cout << p[i] << " " << op[i] << "\n";
//~ }
vector<int>vis(n + 5), dp(n + 5);
auto ans = [&](auto self, int x, int pr) -> void {
dp[x] = 0;
vector<int>io;
for(auto [to, ind] : g[x]){
if(to == pr || vis[ind])continue;
self(self, to, x);
io.push_back(dp[to]);
}
sort(io.begin(), io.end());
reverse(io.begin(), io.end());
int mx = 0;
for(int i = 0;i<io.size();i++){
mx = max(mx, io[i] + i + 1);
}
dp[x] = mx;
};
auto f = [&](int ind) -> int {
vis[ind] = 1;
ans(ans, a, -1);
//~ for(int i = 0;i<n;i++)cout << dp[i] << " ";
//~ cout << "\n";
ans(ans, b, -1);
//~ for(int i = 0;i<n;i++)cout << dp[i] << " ";
//~ cout << "\n";
vis[ind] = 0;
return max(dp[a], dp[b]);
};
int l = 0, r = u.size() - 1, res = -1;
while(r-l >= 75){
int l1 = l + (r-l) / 3;
int r1 = r - (r-l) / 3;
int val1 = f(u[l1]);
int val2 = f(u[r1]);
if(val1 > val2){
l = l1;
}
else r = r1;
}
res = inf;
for(int i = l;i<=r;i++){
int val = f(u[i]);
res = min(res, val);
}
cout << res << "\n";
}
/*
17 1 2
1 3
1 4
4 6
6 7
3 8
3 9
3 10
1 13
13 5
13 11
13 12
13 14
14 15
15 16
15 17
14 2
//
10 1 2
1 2
2 5
1 3
1 4
4 6
6 7
3 8
3 9
3 10
//
6 2 1
1 2
2 3
2 4
1 5
5 6
*/
signed main()
{
//~ freopen("cbarn2.in", "r", stdin);
//~ freopen("cbarn2.out", "w", stdout);
ios_base::sync_with_stdio(0);cin.tie(NULL);cout.tie(NULL);
int tt=1;
//cin >> tt;
while(tt--)solve();
}
