#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#define _GLIBCXX_DEBUG
using namespace std;
using std::cout;
using std::cin;
using std::endl;
using ll=long long;
using ld=long double;
ll ILL=1167167167167167167;
const int INF=2100000000;
const ll mod=1e9+7;
#define rep(i,a) for (ll i=0;i<a;i++)
template<class T> using _pq = priority_queue<T, vector<T>, greater<T>>;
template<class T> ll LB(vector<T> &v,T a){return lower_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> ll UB(vector<T> &v,T a){return upper_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> bool chmin(T &a,const T &b){if(a>b){a=b;return 1;}else return 0;}
template<class T> bool chmax(T &a,const T &b){if(a<b){a=b;return 1;}else return 0;}
template<class T> void So(vector<T> &v) {sort(v.begin(),v.end());}
template<class T> void Sore(vector<T> &v) {sort(v.begin(),v.end(),[](T x,T y){return x>y;});}
void yneos(bool a){if(a) cout<<"Yes\n"; else cout<<"No\n";}


void solve();
// oddloop
int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	
	int t=1;
	//cin>>t;
	rep(i,t) solve();
}

void solve(){
	int N,M,A,B;
	cin>>N>>M>>A>>B;
	vector<vector<int>> G(N+1);
	rep(i,M){
		int a,b;
		cin>>a>>b;
		a--;
		G[a].push_back(b);
		G[b].push_back(a);
	}
	vector<int> seen(N+1,INF);
	queue<int> q;
	rep(i,A) seen[i]=0,q.push(i);
	while(!q.empty()){
		int a=q.front();
		q.pop();
		for(auto x:G[a]){
			if(seen[x]==INF){
				seen[x]=seen[a]+1;
				q.push(x);
			}
		}
	}
	int ans=INF;
	for(int i=B;i<=N;i++) chmin(ans,seen[i]);
	if(ans==INF) cout<<"-1\n";
	else cout<<ans<<"\n";
}