#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define ALL(v) v.begin(), v.end()
typedef long long ll;

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

const ll INF=1LL<<60;

struct Edge{
  int to;
  ll w;
  Edge(int to,ll w) : to(to),w(w) {}
};

using Graph=vector<vector<Edge>>;
using pli=pair<ll,int>;

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

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

  int n,m,a,b;
  cin>>n>>m>>a>>b;
  
  Graph G(100100);
  rep(i,m){
    int l,r;
    cin>>l>>r;
    G[l].push_back(Edge(r+1,1));
    G[r+1].push_back(Edge(l,1));
  }
  
  vector<ll> dist(100100,INF);
  int s=0,t=100010;
  dist[s]=0;
  for(int i=1;i<=a;i++) G[s].push_back(Edge(i,0));
  for(int i=b+1;i<=100001;i++) G[i].push_back(Edge(t,0));
  
  priority_queue<pli,vector<pli>,greater<pli>> que;
  que.push({dist[s],s});
  
  while(!que.empty()){
    int v=que.top().second;
    ll d=que.top().first;
    que.pop();
    
    if(d>dist[v]) continue;
    
    for(auto e:G[v]){
      if(chmin(dist[e.to],dist[v]+e.w)){
        que.push({dist[e.to],e.to});
      }
    }
  }
  if(dist[t]==INF) cout<<-1<<endl;
  else cout<<dist[t]<<endl;

  return 0;
}