import sys
input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
#######################################################################
from heapq import *
INF = 10**18
def dijkstra(g, s, n):
    dist = [INF]*n
    hq = [(0, s)]
    dist[s] = 0
    while hq:
        d, v = heappop(hq)
        if dist[v]!=d:
            continue
        for to, c in g[v]:
            if dist[v] + c < dist[to]:
                dist[to] = dist[v] + c
                heappush(hq, (dist[to], to))
    return dist

n, m, s, t = na()
s -= 1
t -= 1
g = [[] for i in range(n)]
for i in range(m):
    a, b = na()
    a -= 1
    b -= 1
    g[a].append((b, i))
    g[b].append((a, i))

deg = [len(g[i]) for i in range(n)]
q = []
removed = [0] * n
for i in range(n):
    if deg[i] == 1:
        q.append(i)
        removed[i] = 1

f = [0] * m

while q:
    x = q.pop()
    # print(x)
    for y, i in g[x]:
        f[i] = 1
        if removed[y]:
            continue
        # print(x, y, i)
        deg[y] -= 1
        if deg[y] == 1:
            removed[y] = 1
            q.append(y)

BIG = 10 ** 9
G = [[] for i in range(n)]
for x in range(n):
    for y, i in g[x]:
        if f[i] == 0:
            G[x].append((y, BIG - 1))
        else:
            G[x].append((y, BIG))
# print(dist)
dist = dijkstra(G, s, n)
# print(dist)
# print(f)
res = dist[t]
if res == INF:
    print(-1)
else:

# print(dist)

    print((-res) % BIG)