"""

https://yukicoder.me/problems/no/1244

ダイクストラ
右に進む = 黒に塗る
左に進む = 白で塗りなおす
で解けるはず

"""

from sys import stdin
import sys
from collections import deque

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

lis = [ [] for i in range(10**5+10) ]

for i in range(M):

    l,r = map(int,stdin.readline().split())
    lis[l].append(r+1)
    lis[r+1].append(l)

d = [float("inf")] * (10**5+10)

q = deque([])
for i in range(A+1):
    if len(lis[i]) > 0:
        q.append(i)
        d[i] = 0

while len(q):

    now = q.popleft()

    for nex in lis[now]:
        if d[nex] > d[now] + 1:
            d[nex] = d[now] + 1
            q.append(nex)

ans = min(d[B+1:])

if ans == float("inf"):
    print (-1)
else:
    print (ans)