n,m,a,b=map(int,input().split())
lr=[list(map(int,input().split())) for _ in range(m)]
g=[[] for _ in range(n+2)]
for l,r in lr:
  r+=1
  g[l].append(r)
  g[r].append(l)
from heapq import heappush,heappop
inf=float('inf')
seen=[inf]*(n+2)
ans=inf
todo0=[]
for v0 in range(a+1):
  for nv in g[v0]:
    if nv>a:
      if seen[nv]==inf:
        todo0.append(nv)
        seen[nv]=1
seen=[inf]*(n+2)
while todo0:
  v0=todo0.pop()
  todo=[[1,v0]]
  seen[v0]=1
  while todo:
    c,v=heappop(todo)
    if v>b:
      ans=min(ans,c)
      continue
    if seen[v]!=c:continue
    for nv in g[v]:
      if a<nv and seen[nv]>c+1:
        seen[nv]=c+1
        heappush(todo,[c+1,nv])
print(-1 if ans==inf else ans)