#include <bits/stdc++.h>
#define REP(i, n) for (int i = 0; (i) < (int)(n); ++(i))
#define REP3(i, m, n) for (int i = (m); (i) < (int)(n); ++(i))
#define REP_R(i, n) for (int i = (int)(n)-1; (i) >= 0; --(i))
#define REP3R(i, m, n) for (int i = (int)(n)-1; (i) >= (int)(m); --(i))
#define ALL(x) ::std::begin(x), ::std::end(x)
using namespace std;

int64_t solve(int n, int m, int64_t a, int64_t b, const vector<int64_t>& l, const vector<int64_t>& r) {
    vector<vector<int> > g(n + 1);
    REP (j, m) {
        g[l[j]].push_back(r[j]);
        g[r[j]].push_back(l[j]);
    }

    // 0/1-bfs
    vector<int> dist(n + 1, INT_MAX);
    queue<int> que;
    REP (x, n + 1) {
        if (x <= a) {
            dist[x] = 0;
            que.push(x);
        }
    }
    while (not que.empty()) {
        int x = que.front();
        que.pop();
        if (b <= x) {
            return dist[x];
        }
        for (int y : g[x]) {
            if (dist[y] == INT_MAX) {
                dist[y] = dist[x] + 1;
                que.push(y);
            }
        }
    }
    return -1;
}

// generated by online-judge-template-generator v4.7.1 (https://github.com/online-judge-tools/template-generator)
int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    constexpr char endl = '\n';
    int N, M;
    int64_t A, B;
    cin >> N >> M;
    vector<int64_t> L(M), R(M);
    cin >> A >> B;
    -- A;
    REP (i, M) {
        cin >> L[i] >> R[i]; -- L[i];
    }
    auto ans = solve(N, M, A, B, L, R);
    cout << ans << endl;
    return 0;
}