#include<bits/stdc++.h>
using namespace std;
#include <atcoder/all>
using namespace atcoder;
using mint=modint998244353; //1000000007;
using ll=long long;
using pp=pair<ll,ll>;
#define sr string 
#define vc vector
#define fi first
#define se second
#define rep(i,n) for(ll i=0;i<(ll)n;i++)
#define pb push_back
#define all(v) v.begin(),v.end()
#define pque priority_queue
#define bpc(a) __builtin_popcount(a)
int main(){
  int n,m,a,b;cin>>n>>m>>a>>b;
  vc v(n+1,vc<int>(0));
  rep(i,m){
    int x,y;cin>>x>>y; x--;
    v[x].pb(y); v[y].pb(x);
  }
  vc<int>d(n+1,-1);
  rep(i,a)d[i]=0;
  queue<int>q; 
  rep(i,a)q.push(i);
  while(q.size()){
    int now=q.front(); q.pop();
    for(auto au:v[now])if(d[au]==-1){
      q.push(au);
      d[au]=d[now]+1;
    }
  }
  int ans=1e9;
  for(int i=b;i<=n;i++)if(d[i]!=-1)ans=min(ans,d[i]);
  if(ans==1e9)cout<<-1;
  else cout<<ans;
}