#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...));
}
vector<ll>dist(200001);
using P=pair<ll,ll>;
void bfs(ll k,vector<vector<ll>>&g){
  queue<P> q;
  ll n=g.size();
  q.emplace(k,0);
  fill(dist.begin(),dist.begin()+n,INF);
  while(!q.empty()){
    ll x=q.front().first;
    ll len=q.front().second;
    if(dist[x]==INF){
      dist[x]=len;
      for(ll i=0;i<g[x].size();i++){
        q.emplace(g[x][i],len+1);
      }
    }
    q.pop();
  }
}
int main(){
	int n,m,a,b;cin>>n>>m>>a>>b;
	a--;b--;
	vector<vector<ll>>g(n+1);
	rep(i,0,m){
		int l,r;
		cin>>l>>r;
		l--;
        chmax(l,a);
        chmax(r,a);
        chmin(l,b+1);
        chmin(r,b+1);
		g[l].push_back(r);
		g[r].push_back(l);
	}
    bfs(a,g);
	if(dist[b+1]!=INF)cout<<dist[b+1]<<endl;
	else cout<<-1<<endl;
  return 0;
}