#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 << "\n";
  for (int i = 1; i <= n; i++) {
    for (int j = i + 1; j <= i + k; j++) {
      cout << i << " " << (j - 1) % n + 1 << "\n";
    }
  }
}
