n, u, v = map(int, input().split())
u -= 1
v -= 1
m_mat = [[] for i in range(n)]
for _ in range(n-1):
    a, b = map(int, input().split())
    a -= 1
    b -= 1
    m_mat[a].append(b)
    m_mat[b].append(a)
u_map = [-1]*n
v_map = [-1]*n
u_map[u] = 0
v_map[v] = 0
def dfs(current, depth, ma):
    for nex in m_mat[current]:
        if ma[nex] > -1:
            continue
        ma[nex] = depth
        dfs(nex, depth+1, ma)
dfs(u, 1, u_map)
dfs(v, 1, v_map)


ans = max(v_map)-1

print(ans)
