n,m,a,b=map(int,input().split())
lr=[list(map(int,input().split())) for _ in range(m)]
g=[[] for _ in range(n+3)]
a,b=a-1,b
for l,r in lr:
  l,r=l-1,r
  if r<=a or b<=l:
    continue
  if l<=a and b<=r:
    print(1)
    exit()
  elif a<l<=r<b:
    g[l].append(r)
    g[r].append(l)
  elif l<=a<=r:
    g[n+1].append(r)
    g[r].append(n+1)
  else:
    g[n+2].append(l)
    g[l].append(n+2)
#print(g)
from collections import deque
seen=[-1]*(n+3)
todo=deque([n+1])
seen[n+1]=0
while todo:
  v=todo.popleft()
  for nv in g[v]:
    if seen[nv]==-1:
      seen[nv]=seen[v]+1
      todo.append(nv)
print(seen[n+2])
#print(seen)