from collections import deque

n,m,a,b=map(int,input().split())
g=[[] for _ in range(n+2)]
q=deque([])
for _ in range(m):
  l,r=map(int,input().split())
  g[l].append(r+1)
  g[r+1].append(l)
  if l<=a:
    q.append((0,l))

d=[-1]*(n+2)
while q:
  dd,i=q.popleft()
  if 0<=d[i]<=dd:
    continue
  d[i]=dd
  for j in g[i]:
    if d[j]==-1:
      q.append((dd+1,j))

ans=-1
for i in range(b+1,n+2):
  if d[i]>=-1:
    if ans==-1:
      ans=d[i]
    ans=min(ans,d[i])

print(ans)