from collections import deque
import sys
input = sys.stdin.buffer.readline
sys.setrecursionlimit(10 ** 7)
inf = 10**9

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

d = [inf] * (N + 3)
d[source] = 0
q = deque([source])
while q:
    s = q.popleft()
    ds = d[s]
    for t, dt in G[s]:
        if d[t] >= inf:
            if dt == 0:
                d[t] = ds
                q.appendleft(t)
            else:
                d[t] = ds + dt
                q.append(t)

ans = d[sink]
if ans >= inf:
    ans = -1
print(ans)