#include <bits/stdc++.h>
using namespace std;
int mas[1010][1010];
int main() {
  int N, K;
  cin >> N >> K;
  for (int i = 1; i <= N; i++) {
    int k = 0;
    for (int j = 1; j <= N; j++) {
      if (k == K)
        break;
      else if (i == j)
        continue;
      else if (mas[j][i] || mas[i][j])
        continue;
      else {
        mas[i][j] = 1;
        k++;
      }
    }
    if (k < K) {
      cout << -1 << endl;
      return 0;
    }
  }
  int cnt = 0;
  for (int i = 1; i <= N; i++) {
    for (int j = 1; j <= N; j++) {
      if (mas[i][j]) cnt++;
    }
  }
  cout << cnt << endl;
  for (int i = 1; i <= N; i++) {
    for (int j = 1; j <= N; j++) {
      if (mas[i][j]) cout << i << " " << j << endl;
    }
  }
  return 0;
}
