#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  if (2 * k >= n) {
    cout << -1;
    return 0;
  }
  cout << n * k << endl;
  for (int i = 1; i <= n; ++i)
    for (int j = 0; j < k; ++j) printf("%d %d\n", i, (i + j) % n + 1);
}
