#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <cmath>
#include <map>
#include <queue>
#include <iomanip>
#include <set>
#include <tuple>
#define mkp make_pair
#define mkt make_tuple
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define all(v) v.begin(),v.end()
using namespace std;
typedef long long ll;
const ll MOD=1e9+7;
template<class T> void chmin(T &a,const T &b){if(a>b) a=b;}
template<class T> void chmax(T &a,const T &b){if(a<b) a=b;}

int main(){
  cin.tie(0);
  ios::sync_with_stdio(false);

  int N,M,A,B;
  cin>>N>>M>>A>>B;
  vector<int> L(M),R(M);
  rep(i,M) cin>>L[i]>>R[i];

  rep(i,M){
    L[i]--;R[i]--;
  }
  A--;B--;

  vector<vector<int>> g(N+1);
  rep(i,M){
    g[L[i]].push_back(R[i]+1);
    g[R[i]+1].push_back(L[i]);
  }

  queue<int> Q;
  vector<int> dist(N+1,MOD);
  for(int i=0;i<=A;i++){
    dist[i]=0;
    Q.push(i);
  }
  while(!Q.empty()){
    int now=Q.front();
    Q.pop();

    for(auto nex:g[now]){
      if(dist[nex]==MOD){
        dist[nex]=dist[now]+1;
        Q.push(nex);
      }
    }
  }

  int ans=MOD;
  for(int i=B+1;i<=N;i++) chmin(ans,dist[i]);
  if(ans==MOD) cout<<-1<<endl;
  else cout<<ans<<endl;

  return 0;
}