import sys
input = sys.stdin.readline

from collections import deque
inf = 10 ** 18
def bfs(G, s, N):
    Q = deque([])
    dist = [inf] * N
    par = [inf] * N
    for i in range(s):
        dist[i] = 0
        Q.append(i)

    while Q:
        u = Q.popleft()
        for v in G[u]:
            if dist[v] != inf:
                continue
            dist[v] = dist[u] + 1
            par[v] = u
            Q.append(v)
            
    return dist

N, M, A, B = map(int, input().split())
G = [[] for i in range(N + 1)]
for i in range(M):
    L, R = map(int, input().split())
    L -= 1
    G[L].append(R)
    G[R].append(L)

D = bfs(G, A, N + 1)
ans = min(D[B:])
print(ans) if ans != inf else print(-1)