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

typedef long long ll;
#define REP(i,n) for(int i=0,_n=(int)(n);i<_n;++i)
#define ALL(v) (v).begin(),(v).end()
#define CLR(t,v) memset(t,(v),sizeof(t))
template<class T1,class T2>ostream& operator<<(ostream& os,const pair<T1,T2>&a){return os<<"("<<a.first<<","<<a.second<< ")";}
template<class T>void chmin(T&a,const T&b){if(a>b)a=b;}
template<class T>void chmax(T&a,const T&b){if(a<b)a=b;}
#ifdef LOCAL
template<class T>void pv(T a,T b){for(T i=a;i!=b;++i)cerr<<(*i)<<" ";cerr<<endl;}
#else
template<class T>void pv(T a,T b){}
#endif

ll nextLong() { ll x; scanf("%lld", &x); return x;}


int main2() {
  int N = nextLong();
  int M = nextLong();
  int A = nextLong();
  int B = nextLong();

  vector<vector<int>> g(N+3);

  REP(i, M) {
    int L = nextLong();
    int R = nextLong();
    g[L].push_back(R+1);
    g[R+1].push_back(L);
  }

  for (int a = 1; a <= A; a++) g[0].push_back(a);
  for (int b = B+1; b <= N+1; b++) g[b].push_back(N+2);
  
  const int INF = 1001001001;
  vector<int> memo(g.size(), INF);

  queue<int> q;
  q.push(0);
  memo[0] = 0;
  while (!q.empty()) {
    int cur = q.front(); q.pop();
    for (int nxt : g[cur]) {
      if (memo[nxt] > memo[cur] + 1) {
        memo[nxt] = memo[cur] + 1;
        q.push(nxt);
      }
    }
  }
  int ans = memo[N+2] - 2;
  if (ans > INF / 2) ans = -1;
  cout << ans << endl;
  return 0;
}

int main() {

#ifdef LOCAL
  for (;!cin.eof();cin>>ws)
#endif
    main2();
  return 0;
}