import sys
import io, os
input = io.BytesIO(os.read(0,os.fstat(0).st_size)).readline

from collections import deque
INF = 10**18

n, m, a, b = map(int, input().split())
edge = [[] for i in range(n+2)]
for i in range(m):
    l, r = map(int, input().split())
    r += 1
    l = max(l, a)
    r = min(r, b+1)
    edge[l].append(r)
    edge[r].append(l)

q = deque([])
q.append(a)
dist = [INF]*(n+2)
dist[a] = 0
while q:
    v = q.popleft()
    for u in edge[v]:
        if dist[u] != INF:
            continue
        dist[u] = dist[v]+1
        q.append(u)

ans = dist[b+1]
if ans < INF:
    print(ans)
else:
    print(-1)