#include <stdio.h>
#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000300

int main(){	

	int N,M,A,B;
	cin>>N>>M>>A>>B;
	vector<pair<int,int>> P(M);
	vector<vector<int>> E(N+1,vector<int>());
	rep(i,M){
		cin>>P[i].second>>P[i].first;
		P[i].second--;
		E[P[i].second].push_back(P[i].first);
		E[P[i].first].push_back(P[i].second);
	}
	
	vector<int> dis(N+1,Inf);
	queue<int> Q;
	rep(i,A){
		dis[i] = 0;
		Q.push(i);
	}
	
	while(Q.size()!=0){
		int u = Q.front();
		Q.pop();
		rep(i,E[u].size()){
			int v = E[u][i];
			if(dis[v]!=Inf)continue;
			dis[v] = dis[u]+1;
			Q.push(v);
		}
	}
	int ans  =Inf;
	for(int i=B;i<dis.size();i++){
		ans = min(ans,dis[i]);
	}

	if(ans==Inf)cout<<-1<<endl;
	else cout<<ans<<endl;
	
    return 0;
}