n,m,a,b=map(int,input().split())
lr=[list(map(int,input().split())) for _ in range(m)]
g=[set() for _ in range(n+2)]
for l,r in lr:
  r+=1
  l=max(l,a)
  l=min(b+1,l)
  r=max(r,a)
  r=min(b+1,r)
  g[l].add(r)
  g[r].add(l)
inf=float('inf')
seen=[inf]*(n+2)
ans=inf
from collections import deque
todo=deque([a])
seen[a]=0
while todo:
  v=todo.popleft()
  if v>b:
    ans=min(ans,seen[v])
    continue
  nc=seen[v]+1
  for nv in g[v]:
    if seen[nv]>nc:
      seen[nv]=nc
      todo.append(nv)
print(-1 if ans==inf else ans)