#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
#define ll long long
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define repl(i,a,b) for(ll i=(a);i<(b);i++)
#define all(a) (a).begin(),(a).end()
#define rall(a) (a).rbegin(),(a).rend()

template <typename T> bool chmin(T &a,T b){if(a>b){a=b;return true;} return false;}
template <typename T> bool chmax(T &a,T b){if(a<b){a=b;return true;} return false;}


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

  int n,m,s,g; cin >> n >> m >> s >> g;
  s--,g--;
  vector<vector<int>> to(n);
  vector<pair<int,int>> edge;
  rep(i,0,m){
    int u,v; cin >> u >> v;
    u--,v--;
    to[u].push_back(v);
    to[v].push_back(u);
    edge.push_back({u,v});
  }
  vector<int> ord(n,-1),low(n);
  int num=0;
  auto dfs=[&](auto dfs,int pr,int v) -> void{
    ord[v]=num;
    num++;
    low[v]=ord[v];
    for(auto nv:to[v]){
      if(nv == pr || ord[nv] == -1) continue;
      chmin(low[v],ord[nv]);
    }
    for(auto nv:to[v]){
      if(nv == pr || ord[nv] != -1) continue;
      dfs(dfs,v,nv);
      chmin(low[v],low[nv]);
    }
  };
  dfs(dfs,-1,s);
  // rep(i,0,n){
  //   cout << ord[i] << " " << low[i] << '\n';
  // }
  vector<vector<pair<int,int>>> G(n);
  rep(i,0,m){
    auto [u,v]=edge[i];
    if(ord[u]>ord[v]) swap(u,v);
    if(ord[u]<low[v]){
      // cout << i << ":" << 1 << "\n";
      G[u].push_back({v,1});
      G[v].push_back({u,1});
    }
    else{
      // cout << i << ":" << 0 << "\n";
      G[u].push_back({v,0});
      G[v].push_back({u,0});
    }
  }
  int INF=1e9;
  vector<int> dist(n,INF);
  queue<int> que;
  que.push(g);
  dist[g]=0;
  while(!que.empty()){
    auto v=que.front();
    que.pop();
    for(auto nv:to[v]){
      if(chmin(dist[nv],dist[v]+1)) que.push(nv);
    }
  }
  if(dist[s] == INF){
    cout << -1 << '\n';
    return 0;
  }
  deque<int> dq;
  dq.push_back(s);
  vector<int> d(n,INF);
  d[s]=0;
  while(!dq.empty()){
    auto v=dq.front();
    dq.pop_front();
    for(auto [nv,cost]:G[v]){
      if(dist[nv] != dist[v]-1) continue;
      if(cost == 0){
        if(chmin(d[nv],d[v])) dq.push_front(nv);
      }
      else{
        if(chmin(d[nv],d[v]+1)) dq.push_back(nv);
      }
    }
  }
  cout << dist[s]-d[g] << '\n';
}