import sys
input = sys.stdin.readline

from collections import deque

N,M,A,B=map(int,input().split())

X=[]

for i in range(M):
    l,r=map(int,input().split())

    l=max(l,A)
    r=min(r,B)

    X.append((l,r))

E=[[] for i in range(N+1)]

for x,y in X:
    E[x-1].append(y)
    E[y].append(x-1)

DIS=[1<<60]*(N+1)

DIS[A-1]=0
Q=deque()
Q.append(A-1)

while Q:
    x=Q.popleft()

    for to in E[x]:
        if DIS[to]>DIS[x]+1:
            DIS[to]=DIS[x]+1
            Q.append(to)

ANS=DIS[B]

if ANS>1000000:
    print(-1)
else:
    print(ANS)