from collections import deque


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

G = [list() for _ in range(N+2)]
for i in range(M):
    l,r = map(int, input().split())
    if (l<=A and r<=A) or (l>=B and r>=B):continue
    if l<=A:
        l = A
    if r>=B:
        r = B
    G[l].append(r+1)
    G[r+1].append(l)

dis = [-1]*(N+2)
dis[A] = 0
d = deque()
d.append(A)
while d:
    v = d.popleft()
    for nex in G[v]:
        if dis[nex]==-1:
            dis[nex] = dis[v]+1
            d.append(nex)
print(dis[B+1])