#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;

//template
#define rep(i,a,b) for(int i=(int)(a);i<(int)(b);i++)
#define ALL(v) (v).begin(),(v).end()
using ll=long long int;
const int inf = 0x3fffffff; const ll INF = 0x1fffffffffffffff; const double eps=1e-12;
template<typename T>inline bool chmax(T& a,T b){if(a<b){a=b;return 1;}return 0;}
template<typename T>inline bool chmin(T& a,T b){if(a>b){a=b;return 1;}return 0;}



int main(){
   int n,m,a,b;
   cin>>n>>m>>a>>b;
   a--;
   vector g(n+1,vector<int>());
   vector<int> dist(n+1,inf);
   queue<int> que;
   rep(i,0,a+1){
      dist[i]=0;
      que.push(i);
   }
   rep(_,0,m){
      int x,y;
      cin>>x>>y;
      x--;
      g[x].push_back(y);
      g[y].push_back(x);
   }

   while(!que.empty()){
      int v=que.front();
      que.pop();
      if(b<=v){
         cout<<dist[v]<<'\n';
         return 0;
      }
      for(auto& to:g[v])if(chmin(dist[to],dist[v]+1)){
         que.push(to);
      }
   }
   puts("-1");
   return 0;
}