#https://tjkendev.github.io/procon-library/python/graph/bridge.html
def bridge(G, N):
    result = set()
    label = [None]*N
    gen = 0
    cost = [0]*N
    def dfs(u, p):
        nonlocal gen
        res = 0
        for v, _ in G[u]:
            if v == p:
                continue
            if label[v] is not None:
                if label[v] < label[u]:
                    cost[v] += 1
                    res += 1
            else:
                label[v] = gen; gen += 1
                r = dfs(v, u)
                if r == 0:
                    result.add(_)
                res += r
        res -= cost[u]
        return res
    for v in range(N):
        if not label[v]:
            label[v] = gen; gen += 1
            r = dfs(v, -1)
            assert r == 0, r
    return result

n, m, s, t = list(map(int, input().split()))
s, t = s-1, t-1
node = [[] for _ in range(n)]
E = [1] * m
for _ in range(m):
    u, v = list(map(lambda x: int(x)-1, input().split()))
    node[u].append((v, _))
    node[v].append((u, _))

result = bridge(node, n)
for i in result:
    E[i] = 0

D = [-1 for _ in range(n)]
F = [-1] * n
B = [s]
D[s] = 0
F[s] = 0
while A := B:
    B = []
    for now in A:
        cnt = F[now]
        for nxt, idx in node[now]:
            ncnt = cnt + E[idx]
            if D[nxt] == -1:
                D[nxt] = D[now] + 1
                B.append(nxt)
                F[nxt] = ncnt
            elif D[nxt] == D[now] + 1:
                F[nxt] = max(F[nxt], ncnt)
print(F[t])