from collections import deque
N,M,A,B=map(int,input().split())
G=[[] for _ in range(N+1)]
for _ in range(M):
  l,r=map(int,input().split())
  G[l-1].append(r)
  G[r].append(l-1)
D=deque()
depth=[-1]*(N+1)
for i in range(A):
  D.append(i)
  depth[i]=0
while D:
  v=D.popleft()
  for nv in G[v]:
    if depth[nv]==-1:
      depth[nv]=depth[v]+1
      D.append(nv)
ans=10**9
for i in range(B,N+1):
  if depth[i]!=-1:
    ans=min(ans,depth[i])
if ans==10**9:
  print(-1)
else:
  print(ans)