from collections import deque

def bfs(starts):
    dist=[INF]*(N+1)
    next_set=deque()
    for start in starts:
        next_set.append((start,0))
        dist[start]=0
    while next_set:
        p,t=next_set.popleft()
        for q in edges[p]:
            if dist[q]!=INF:
                continue
            dist[q]=t+1
            next_set.append((q,t+1))
    return dist

##############################################

N,M,A,B=map(int, input().split())
A-=1
B-=1
INF=10**18
res=INF
starts=[]
edges=[[] for _ in range(N+1)]
for _ in range(M):
    l,r=map(int, input().split())
    l-=1
    r-=1
    if l<=A:
        starts.append(l)
        edges[l].append(r+1)
    elif A<=l<=B:
        edges[l].append(r+1)
        edges[r+1].append(l)

dist=bfs(starts)

for i in range(B+1,N+1):
    res=min(dist[i],res)


print(res if res!=INF else -1)