N,M,A,B = map(int,input().split())
G = [[] for _ in range(N+1)]
for _ in range(M):
    l,r = map(int,input().split())
    if l <= A:
        G[0].append(r)
    else:
        G[l-1].append(r)
        G[r].append(l-1)
inf = 10 ** 6
dist = [inf] * (N+1)
dist[0] = 0
from collections import deque
q = deque()
q.append(0)
while q:
    now = q.popleft()
    for v in G[now]:
        if dist[v] > dist[now] + 1:
            dist[v] = dist[now] + 1
            q.append(v)
ans = inf
for i in range(B,N+1):
    if dist[i] < ans:
        ans = dist[i]
print(ans if ans != inf else -1)