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

using Int = long long;
const char newl = '\n';

template<typename T1,typename T2> inline void chmin(T1 &a,T2 b){if(a>b) a=b;}
template<typename T1,typename T2> inline void chmax(T1 &a,T2 b){if(a<b) a=b;}
template<typename T> void drop(const T &x){cout<<x<<endl;exit(0);}
template<typename T=int>
vector<T> read(size_t n){
  vector<T> ts(n);
  for(size_t i=0;i<n;i++) cin>>ts[i];
  return ts;
}


template<typename T>
struct Dijkstra{
  struct Edge{
    int to;
    T cost;
    Edge(int to,T cost):to(to),cost(cost){}
    bool operator<(const Edge &o)const{return cost>o.cost;}
  };

  vector< vector<Edge> > G;
  vector<T> ds;
  vector<int> bs;
  Dijkstra(int n):G(n){}

  void add_edge(int u,int v,T c){
    G[u].emplace_back(v,c);
  }

  void build(int s){
    int n=G.size();
    ds.assign(n,numeric_limits<T>::max());
    bs.assign(n,-1);

    priority_queue<Edge> pq;
    ds[s]=0;
    pq.emplace(s,ds[s]);

    while(!pq.empty()){
      auto p=pq.top();pq.pop();
      int v=p.to;
      if(ds[v]<p.cost) continue;
      for(auto e:G[v]){
        if(ds[e.to]>ds[v]+e.cost){
          ds[e.to]=ds[v]+e.cost;
          bs[e.to]=v;
          pq.emplace(e.to,ds[e.to]);
        }
      }
    }
  }

  T operator[](int k){return ds[k];}

  vector<int> restore(int to){
    vector<int> res;
    if(bs[to]<0) return res;
    while(~to) res.emplace_back(to),to=bs[to];
    reverse(res.begin(),res.end());
    return res;
  }
};

//INSERT ABOVE HERE
signed main(){
  cin.tie(0);
  ios::sync_with_stdio(0);

  int n,m,a,b;
  cin>>n>>m>>a>>b;
  a--;

  int S=n+1,T=n+2;
  Dijkstra<int> D(n+3);
  for(int i=0;i<m;i++){
    int l,r;
    cin>>l>>r;
    l--;
    D.add_edge(l,r,1);
    D.add_edge(r,l,1);
  }
  for(int i=0;i<=a;i++)
    D.add_edge(S,i,0);
  for(int i=b;i<=n;i++)
    D.add_edge(i,T,0);
  D.build(S);

  if(~D.bs[T]) drop(D[T]);
  drop(-1);
  return 0;
}