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
todo=[]
for v0 in range(a+1):
  for nv in g[v0]:
    if nv>a:
      if seen[nv]==inf:
        todo.append([1,nv])
        seen[nv]=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)