#include <bits/stdc++.h>
using namespace std;

const int N = 1000010 , inf = 1e8;
int n , B , A;
int head[N] , to[N*2] , nxt[N*2] , cnt;
void addedge(int x , int y) {
	to[++cnt] = y , nxt[cnt] = head[x] , head[x] = cnt;
}

inline int read() {
    int x = 0 ; char c = getchar(); bool f = 0;
    while(c < '0' || c > '9') f |= (c=='-') , c = getchar();
    while(c >= '0' && c <= '9') x = x * 10 + (c^48) , c = getchar();
    return f ? -x : x;
}

int deep[N] , fa[N];
void dfs(int x) {
	for(int i = head[x] ; i ; i = nxt[i]) {
		int y = to[i];
		if(y == fa[x]) continue;
		fa[y] = x , deep[y] = deep[x] + 1 , dfs(y);
	}
}
bool onpath[N] ;
int dp[N];
vector<int> branch[N];

void DP(int x , int top , int sum) {
	if(onpath[x]) top = x;
	int son = 0;
	for(int i = head[x] ; i ; i = nxt[i]) {
		int y = to[i];
		if(y == fa[x]) continue;
		if(son) sum++;
		son++; 
	}
	if(!son) {dp[x] = deep[x] - deep[top] + sum ; return ;}
	
	int maxdp = -inf , max2dp = -inf;
	for(int i = head[x] ; i ; i = nxt[i]) {
		int y = to[i];
		if(y == fa[x]) continue;
		DP(y , top , sum);
		if(dp[y] > maxdp) max2dp = maxdp , maxdp = dp[y];
		else if(dp[y] > max2dp) max2dp = dp[y]; 
	}
	if(son == 1) dp[x] = deep[x] - deep[top] + sum + 1;
	else dp[x] = max2dp;
}

void prepare() {
	n = read() , B = read() , A = read();
	if(A == B) {puts("0"); exit(0);} 
	for(int i = 1 ; i < n ; ++i) {
		int u = read() , v = read();
		addedge(u , v) , addedge(v , u);
	}
	dfs(B);
	for(int p = A ; p != B ; p = fa[p]) onpath[p] = true;
	for(int i = head[B] ; i ; i = nxt[i]) {
		int y = to[i];
		if(onpath[y]) DP(y , y , 0);
	}
	for(int p = A ; p != B ; p = fa[p]) {
		for(int i = head[p] ; i ; i = nxt[i]) {
			int y = to[i];
			if(onpath[y] || y == B) continue;
			branch[p].push_back(dp[y]);
		}
		sort(branch[p].begin() , branch[p].end());
	}
}

bool check(int x) {
	int ban = 0 , id = 0;
	for(int p = A ; p != B ; p = fa[p]) {
		for(int i = 0 ; i < branch[p].size(); ++i)
			if(branch[p][i] + ban > x + (p!=A))
				ban++;
		if(ban > ++id) return false;
	}
	return true;
}

int main() {
//	freopen("fish.in" , "r" , stdin);
//	freopen("fish.out" , "w" , stdout);
	prepare();
	int l = dp[A] , r = 2*n , mid;
	while(l < r) {
		mid = (l + r) >> 1;
		if(check(mid)) r = mid;
		else l = mid + 1;
	}
	printf("%d\n" , l);
	return 0;
}