#include <bits/stdc++.h>
//#include <chrono>
//#pragma GCC optimize("Ofast")
using namespace std;
#define reps(i,s,n) for(int i = s; i < n; i++)
#define rep(i,n) reps(i,0,n)
#define Rreps(i,n,e) for(int i = n - 1; i >= e; --i)
#define Rrep(i,n) Rreps(i,n,0)
#define ALL(a) a.begin(), a.end()
#define fi first
#define se second

using ll = long long;
using vec = vector<ll>;
using mat = vector<vec>;

ll N,M,H,W,Q,K,A,B;
string S;
typedef pair<ll, ll> P;
const ll INF = (1LL<<58);

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

int main() {
    cin>>N>>M>>A>>B;
    mat G(N + 3, vec(0));
    const int st = 0, gl = N + 2;
    rep(i, M){
        int l, r; cin>>l>>r; ++r;
        G[l].push_back(r);
        G[r].push_back(l);
    }
    rep(i, A) G[st].push_back(i + 1);
    reps(i, B + 1, gl) G[i].push_back(gl);
    vec dist(N + 3, INF);
    dist[st] = 0;
    priority_queue<P, vector<P>, greater<> > pque;
    pque.emplace(0, st);
    while(!pque.empty()){
        P p = pque.top(); pque.pop();
        if(dist[p.se] < p.fi) continue;
        for(ll to : G[p.se]){
            if(chmin(dist[to], p.fi + 1)){
                pque.emplace(p.fi + 1, to);
            }
        }
    }
    cout<<(dist[gl] == INF ? -1 : dist[gl] - 2)<<endl;
}