#include <bits/stdc++.h>
using namespace std;
typedef long long int lli;

int main() {

    int Total, N, M; cin >> Total >> N >> M;
    if (N != Total && M != Total) {
        cout << "-1\n";
        return 0;
    }

    if (N == M && (N == 2 || N == 3)) {
        cout << "-1\n";
        return 0;
    }

    if (N != Total) {
        cout << N - 1 << "\n";
        for (int i = 2; i <= N; i++) {
            cout << i - 1 << " " << i << "\n";
        }
    } else {
        int amnt = 0;
        for (int i = 1; i <= Total; i++) {
            for (int k = i + 1; k <= Total; k++) {
                if (i + 1 == k && k <= M) continue;
                amnt++;
            }
        }

        cout << amnt << "\n";
        for (int i = 1; i <= Total; i++) {
            for (int k = i + 1; k <= Total; k++) {
                if (i + 1 == k && k <= M) continue;
                cout << i << " " << k << "\n";
            }
        }
    }

    return 0;
}