import sys

sys.setrecursionlimit(200005)
int1 = lambda x: int(x)-1
p2D = lambda x: print(*x, sep="\n")
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
inf = 10**16
md = 998244353
# md = 10**9+7

from collections import deque

n, m, a, b = LI()

to = [[] for _ in range(n+1)]

a -= 1
s = set()
for _ in range(m):
    l, r = LI()
    l -= 1
    if r <= a or b <= l: continue
    if l <= a: s.add(l)
    to[l].append(r)
    to[r].append(l)

dist = [inf]*(n+1)
q = deque()
for i in s:
    q.append(i)
    dist[i] = 0
while q:
    i = q.popleft()
    d = dist[i]
    for j in to[i]:
        if dist[j] != inf: continue
        dist[j] = d+1
        q.append(j)

ans = min(dist[b:])
if ans == inf: ans = -1
print(ans)