import sys
input = sys.stdin.readline

from collections import deque

V,E,S,G=list(map(int,input().split()))

EDGE=[list(map(int,input().split())) for i in range(E)]

for i in range(E):
    EDGE[i][0]-=1
    EDGE[i][1]-=1

ELIST=[[] for i in range(V)]
ELIST2=[set() for i in range(V)]

for a,b in EDGE:
    ELIST[a].append(b)
    ELIST[b].append(a)

    ELIST2[a].add(b)
    ELIST2[b].add(a)

Q=[]
for i in range(V):
    if len(ELIST2[i])==1:
        Q.append(i)

while Q:
    x=Q.pop()

    for to in ELIST2[x]:
        to2=to

    ELIST2[x].remove(to2)
    ELIST2[to2].remove(x)

    if len(ELIST2[to2])==1:
        Q.append(to2)
        
DP=[[1<<60,0] for i in range(V)]

DP[S-1]=[0,0]
Q=deque()
Q.append(S-1)

while Q:
    x=Q.popleft()
    now,m=DP[x]

    for to in ELIST[x]:
        if to in ELIST2[x]:
            if DP[to][0]>now+1:
                DP[to]=[now+1,m+1]
                Q.append(to)
            elif DP[to][0]==now and DP[to][1]<m+1:
                DP[to]=[now+1,m+1]
                Q.append(to)
        else:
            if DP[to][0]>now+1:
                DP[to]=[now+1,m]
                Q.append(to)
            elif DP[to][0]==now and DP[to][1]<m:
                DP[to]=[now+1,m]
                Q.append(to)
            
            
print(DP[G-1][1])