#include <bits/stdc++.h>
using namespace std;
int arr[1001][1001];
int main() {
  int n, m, f1 = 0;
  cin >> n >> m;
  int cnt = m;
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      if (i != j && arr[i][j] == 0) {
        arr[i][j] = 1;
        arr[j][i] = -1;
        cnt--;
      }
      if (cnt == 0) break;
    }
    if (cnt > 0) {
      f1 = 1;
      break;
    }
    cnt = m;
  }
  if (f1 == 1)
    cout << "-1" << endl;
  else {
    cout << n * m << endl;
    for (int i = 0; i < n; i++) {
      for (int j = 0; j < n; j++)
        if (arr[i][j] == 1) cout << i + 1 << " " << j + 1 << endl;
    }
  }
  return 0;
}
