from collections import deque

n,m,a,b=map(int,input().split())
v=[[] for i in range(n+1)]
for i in range(m):
  l,r=map(int,input().split())
  l-=1
  v[l].append(r);v[r].append(l)
f=deque([i for i in range(a)])
ans=[0 if i<a else -1 for i in range(n+1)]
s=-1
while f:
  q=f.popleft()
  if q>=b:
    s=ans[q]
    break
  for i in v[q]:
    if ans[i]==-1:
      ans[i]=ans[q]+1
      f.append(i)
print(s)