import sys


def dfs(cur, par, g, d):
    pars[cur] = par
    for to in g[cur]:
        if to == par:
            continue
        d[to] = d[cur]+1
        md[cur] = max(md[cur], dfs(to, cur, g, d)+1)
    return md[cur]


sys.setrecursionlimit(1000000)
n, u, v = map(int, input().split())
u -= 1
v -= 1
g = []
for i in range(n):
    g.append([])
for i in range(n-1):
    a, b = map(int, input().split())
    g[a-1].append(b-1)
    g[b-1].append(a-1)
d = []
md = []
pars = []
f = []
for i in range(n):
    d.append(-1)
    md.append(0)
    pars.append(-1)
    f.append(False)
d[u] = 0
dfs(u, -1, g, d)
cur = v
while cur != u:
    f[cur] = True
    cur = pars[cur]
pars[u] = True
res = d[v]//2
for i in range(n):
    if f[i] and d[i]*2 < d[v]:
        for to in g[i]:
            if to == pars[i] or f[to]:
                continue
            res = max(res, md[to]+1+d[v]-d[i] - 1)
print(res)
