#include <bits/stdc++.h>
using namespace std;

void ng() {
    cout << -1 << endl;
    exit(EXIT_SUCCESS);
}

using P = pair<int, int>;

int main() {
    int N, K;
    cin >> N >> K;
    if (K == 0) {
        ng();
    }
    int n = -1;
    vector<P> es;
    if (N % 2 == 0) {
        n = N / 2;
        if (K < n) {
            ng();
        }
        for (int i = 0; i < n; ++i) {
            int v = 2 * i, u = v + 1;
            if (K >= 2) {
                es.emplace_back(v, v);
                es.emplace_back(u, u);
                K -= 2;
            } else {
                es.emplace_back(u, v);
                K -= 1;
            }
        }
        if (K > 0) {
            ng();
        }
    } else {
        n = (N - 3) / 2;
        if (K < n + 2) {
            ng();
        }
        for (int i = 0; i < n; ++i) {
            int u = 2 * i, v = u + 1;
            if (K >= 2) {
                es.emplace_back(u, u);
                es.emplace_back(v, v);
                K -= 2;
            } else {
                es.emplace_back(u, v);
                K -= 1;
            }
        }
        int u = 2 * n, v = u + 1, w = v + 1;
        if (K <= 1) {
            ng();
        } else if (K == 2) {
            es.emplace_back(u, v);
            es.emplace_back(v, w);
        } else if (K == 3) {
            es.emplace_back(u, u);
            es.emplace_back(v, v);
            es.emplace_back(w, w);
        } else {
            ng();
        }
    }
    cout << es.size() << endl;
    for (auto [u, v] : es) {
        cout << u + 1 << " " << v + 1 << endl;
    }
}