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