def calc_destination(start, edges, N):
    destination = [-1] * (N + 1)
    destination[start] = 0
    q = [start]
    while len(q) != 0:
        current = q.pop(0)
        for n in edges[current]:
            if destination[n] != -1:
                continue
            destination[n] = destination[current] + 1
            q.append(n)
    return destination

def main():
    N, u, v = map(int, input().split())

    if u == v:
        print(0)
        return

    edges = [[] for _ in range(N + 1)]
    for _ in range(N - 1):
        A, B = map(int, input().split())
        edges[A].append(B)
        edges[B].append(A)

    tak = calc_destination(u, edges, N)
    aok = calc_destination(v, edges, N)

    result = 0
    for i in range(1, N + 1):
        aoki = aok[i]
        if tak[i] >= aoki:
            continue
        if aoki > result:
            result = aoki
    print(result - 1)

main()
