#include <iostream>
#include <algorithm>
#include <iomanip>
#include <vector>
#include <queue>
#include <set>
#include <map>

using namespace std;
typedef long long ll;

const ll INF = 1e+15;

typedef pair<ll, int> P;

struct edge{
    int to;
    ll cost;
};

vector<edge> G[100005];
ll d[100005];

void dijkstra(int s, int num_v){
    priority_queue<P, vector<P>, greater<P>> que;
    fill(d, d+num_v, INF);
    d[s] = 0;
    que.push(P(0, s));
    while(!que.empty()){
        P p = que.top();que.pop();
        int v = p.second;
        if(d[v] < p.first) continue;
        for(int i = 0; i < G[v].size(); i++){
            edge e = G[v][i];
            if(d[v] + e.cost < d[e.to]){
                d[e.to] = d[v] + e.cost;
                que.push(P(d[e.to], e.to));
            }
        }
    }
}

int N, M, A, B;
int L[100005], R[100005];


int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << setprecision(10) << fixed;
    cin >> N >> M >> A >> B;
    A--;// B--;
    for(int i = 0; i < M; i++){
        cin >> L[i] >> R[i]; L[i]--; R[i]--;
    }
    for(int i = 0; i < A; i++){
        G[i].push_back((edge){i+1, 0});
    }
    for(int i = B; i < N; i++){
        G[i].push_back((edge){i+1, 0});
    }
    for(int i = 0; i < M; i++){
        G[L[i]].push_back((edge){R[i]+1, 1});
        G[R[i]+1].push_back((edge){L[i], 1});
    }
    dijkstra(0, N+1);
    // for(int i = 0; i <= N; i++) cout << d[i] << ' ';
    // cout << endl;
    if(d[N] == INF){
        cout << -1 << endl;
        return 0;
    }
    cout << d[N] << endl;
}