#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>
#include <climits>
#include <cstring>
#include <cassert>


using namespace std;
//using namespace atcoder;

#define REP(i, n) for (int i=0; i<(n); ++i)
#define RREP(i, n) for (int i=(int)(n)-1; i>=0; --i)
#define FOR(i, a, n) for (int i=(a); i<(n); ++i)
#define RFOR(i, a, n) for (int i=(int)(n)-1; i>=(a); --i)

#define SZ(x) ((int)(x).size())
#define ALL(x) (x).begin(),(x).end()

#define DUMP(x) cerr<<#x<<" = "<<(x)<<endl
#define DEBUG(x) cerr<<#x<<" = "<<(x)<<" (L"<<__LINE__<<")"<<endl;

template<class T>
ostream &operator<<(ostream &os, const vector<T> &v) {
    os << "[";
    REP(i, SZ(v)) {
        if (i) os << ", ";
        os << v[i];
    }
    return os << "]";
}

template<class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
    return os << "(" << p.first << " " << p.second << ")";
}

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

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

using ll = long long;
using ull = unsigned long long;
using ld = long double;
using P = pair<int, int>;
using vi = vector<int>;
using vll = vector<ll>;
using vvi = vector<vi>;
using vvll = vector<vll>;

const ll MOD = 1e9 + 7;
const int INF = INT_MAX / 2;
const ll LINF = LLONG_MAX / 2;
const ld eps = 1e-9;


// edit


template<typename T>
struct edge {
    int src, to;
    T cost;
};

template<typename T>
using Graph = vector<vector<edge<T>>>;

template<typename T>
vector<T> dijkstra(const Graph<T> &g, int s, T inf) {
    vector<T> d(g.size(), inf);

    using Pi = pair<T, int>;
    priority_queue<Pi, vector<Pi>, greater<Pi>> que;
    d[s] = 0;
    que.emplace(d[s], s);
    while (!que.empty()) {
        T cost;
        int v;
        tie(cost, v) = que.top();
        que.pop();
        if (d[v] < cost) continue;
        for (auto &e : g[v]) {
            auto nxt = cost + e.cost;
            if (d[e.to] > nxt) {
                d[e.to] = nxt;
                que.emplace(nxt, e.to);
            }
        }
    }
    return d;
}

void solve() {
    int N, M, A, B;
    cin >> N >> M >> A >> B;

//    vector<vector<pair<int, int>>> G(N + 2);
    int S = 0, T = N + 1;
    Graph<int> G(N + 2);
    REP(i, M) {
        int L, R;
        cin >> L >> R;
        R++;
        if (L <= A) {
            G[S].push_back(edge<int>{S, L, 0});
        }
        G[L].push_back(edge<int>{L, R, 1});
        G[R].push_back(edge<int>{R, L, 1});
        if (B < R) {
            G[R].push_back(edge<int>{R, T, 0});
        }
    }

    auto dist = dijkstra(G, S, INF);
    if (dist[T] == INF) {
        cout << "-1" << endl;
    } else {
        cout << dist[T] << endl;
    }


}


int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
//    std::ifstream in("input.txt");
//    std::cin.rdbuf(in.rdbuf());

    solve();


    return 0;
}