import sys

N,M,A,B = map(int,input().split())
LR = [list(map(int,input().split())) for _ in range(M)]
E = [[] for _ in range(B - A + 2)]
N2 = B - A + 1
for i in range(M):
    l,r = LR[i]
    a = max(0,min(B-A+1,l-A))
    b = max(0,min(B-A+1,r-A+1))
    E[a].append((b))
    E[b].append((a))
#print(E)
q1 = {0}
q2 = set()
nvstd = [True] * (N2+1)
nvstd[0] = False
ans = 0
while q1:
    ans += 1
    while q1:
        frm = next(iter(q1))
        q1.remove(frm)
        for to in E[frm]:
            if to == N2:
                print(ans)
                sys.exit()

            if nvstd[to]:
                q2.add(to)
                nvstd[to] = False

    q1,q2 = q2,q1
    q2 = set()

print(-1)