#include <bits/stdc++.h>
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define chmax(a, b) a = max(a, b)
#define chmin(a, b) a = min(a, b)
#define all(x) (x).begin(), (x).end()
using namespace std;
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;

int main() {
    int n, m, a, b;
    cin >> n >> m >> a >> b;
    a--;
    b -= a;
    VVI to(n-a+1);
    rep(_, m) {
        int l, r;
        cin >> l >> r;
        if (r <= a) continue;
        l--;
        if (l <= a) l = 0;
        else l -= a;
        r -= a;
        to[l].push_back(r);
        to[r].push_back(l);
    }
    queue<P> q;
    q.emplace(0, 0);
    vector<bool> visited(n-a+1);
    visited[0] = true;
    while(!q.empty()) {
        int u, k;
        tie(u, k) = q.front(); q.pop();
        for(int v: to[u]) {
            if (visited[v]) continue;
            visited[v] = true;
            if (v >= b) {
                cout << k + 1 << endl;
                return 0;
            }
            q.emplace(v, k+1);
        }
    }
    cout << -1 << endl;
}