from collections import deque

N, M, A, B = map(int, input().split())
G = [[] for _ in range(N + 3)]
for _ in range(M):
    l, r = map(int, input().split())
    r += 1
    G[l].append(r)
    G[r].append(l)
for i in range(1, A + 1):
    G[0].append(i)
for i in range(B + 1, N + 2):
    G[i].append(N + 2)

q = deque([0])
INF = 10 ** 9
dist = [INF] * (N + 3)
dist[0] = 0
while q:
    v = q.popleft()
    dx = dist[v] + 1
    for x in G[v]:
        if dist[x] == INF:
            q.append(x)
            dist[x] = dx
print(-1 if dist[-1] == INF else dist[-1] - 2)