### BUGGY CODE ###

import sys
sys.setrecursionlimit(10**9)
import sys
input = sys.stdin.readline

class LCA1():
    def __init__(self, l, start):
        self.n = len(l)
        self.dep=[0]*self.n
        self.par=[[-1]*self.n for i in range(18)]
        self.is_leaf = [0]*self.n

        def dfs(p,v,d):
            self.dep[v]=d
            self.par[0][v]=p
            f = True
            for w in l[v]:
                if w!=p:
                    dfs(v,w,d+1)
                    f = False
            if f:
                self.is_leaf[v] = 1
        dfs(-1,start,0)
        for i in range(17):
            for j in range(self.n):
                self.par[i+1][j]=self.par[i][self.par[i][j]]

    def lca(self,a,b):
        if self.dep[a]>self.dep[b]:
            a,b=b,a
        for i in range(18):
            if (self.dep[b]-self.dep[a]) & 1<<i:
                b=self.par[i][b]
        if a==b:
            return a
        for i in range(18)[::-1]:
            if self.par[i][a]!=self.par[i][b]:
                a=self.par[i][a]
                b=self.par[i][b]
        return self.par[0][a]

    def dist(self, a,b):
        return abs(self.dep[a]-self.dep[b])

def main():
    n, u, v = map(int, input().split())
    u, v = u-1, v-1
    G = [[] for i in range(n)]
    for i in range(n-1):
        a,b = map(int, input().split())
        a,b = a-1,b-1
        G[a].append(b)
        G[b].append(a)

    LCA = LCA1(G, v)
    ans = 0
    for i in range(n):
        if not LCA.is_leaf[i]:
            continue
        lca = LCA.lca(u, i)
        if lca == v:
            continue
        if LCA.dist(lca, u) >= LCA.dist(lca, v):
            continue
        tmp = 0
        d = LCA.dist(u, lca) + LCA.dist(lca, i)
        if d != 0:
            tmp = 2*d
            aoki = d
        else:
            aoki = 0
        tak = LCA.dep[i]
        m = tak-aoki
        if m%2==1:
            tmp += 2*m -1
        else:
            tmp += 2*(m-1)
        ans = max(ans, tmp)
    print(ans//2)

main()

### FIXED CODE ###

import sys
input = sys.stdin.readline
sys.setrecursionlimit(10**9)

from collections import deque

def main():
    n, u, v = map(int, input().split())
    u, v = u-1, v-1
    G = [[] for i in range(n)]
    for i in range(n-1):
        a,b = map(int, input().split())
        a,b = a-1,b-1
        G[a].append(b)
        G[b].append(a)

    def bfs(start):
        dist = [0]*n
        is_leaf = [0]*n
        que = deque()
        que.append((start, -1, 0))
        while que:
            c,p,d = que.pop()
            dist[c] = d
            cnt = 0
            for to in G[c]:
                if to != p:
                    que.append((to,c,d+1))
                    cnt += 1
            if not cnt:
                is_leaf[c] = 1
        return dist, is_leaf

    a, is_leaf = bfs(v)
    b, _ = bfs(u)

    ans = 0
    for i in range(n):
        if not is_leaf[i]:
            continue
        if b[i] >= a[i]:
            continue
        tmp = 0
        d = b[i]
        if d != 0:
            tmp = 2*d
            aoki = d
        else:
            aoki = 0
        tak = a[i]
        m = tak-aoki
        if m%2==1:
            tmp += 2*m -1
        else:
            tmp += 2*(m-1)
        ans = max(ans, tmp)
    print(ans//2)

if __name__ == "__main__":
    main()