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

#define ALL(a)  (a).begin(),(a).end()
#define ALLR(a)  (a).rbegin(),(a).rend()
#define rep(i,n,m) for(int i = (n); i < (int)(m); i++)
#define rrep(i,n,m) for(int i = (m) - 1; i >= (int)(n); i--)
using ll = long long;
using ld = long double;
const ll MOD = 1e9+7;
//const ll MOD = 998244353;
const ll INF = 1e9+1;
template<typename T>
bool chmin(T &a,T b){if(a>b){a=b;return true;}else return false;}
template<typename T>
bool chmax(T &a,T b){if(a<b){a=b;return true;}else return false;}
template<typename T>
vector<ll>dx={1,0,-1,0,1,1,-1,-1};
vector<ll>dy={0,1,0,-1,1,-1,1,-1};
template<typename T>
vector<T> make_v(size_t a,T b){return vector<T>(a,b);}
template<typename... Ts>
auto make_v(size_t a,Ts... ts){
  return vector<decltype(make_v(ts...))>(a,make_v(ts...));
}
int main(){
	int n,m,a,b;cin>>n>>m>>a>>b;
	a--;b--;
	vector<vector<int>>g(n+1);
	rep(i,0,m){
		int l,r;
		cin>>l>>r;
		l--;
		g[l].push_back(r);
		g[r].push_back(l);
	}
	queue<pair<int,int>>que;
	vector<int>dist(n+1,INF);
	rep(i,0,a+1)que.emplace(i,0);
	while(!que.empty()){
		auto now=que.front();
		que.pop();
		if(dist[now.first]!=INF)continue;
		dist[now.first]=now.second;
		for(auto to:g[now.first]){
			que.emplace(to,now.second+1);
		}
	}
	int res=INF;
	rep(i,b+1,n+1)chmin(res,dist[i]);
	if(res!=INF)cout<<res<<endl;
	else cout<<-1<<endl;
  return 0;
}