#include <bits/stdc++.h>
using namespace std;
int main(){
  int N, K;
  cin >> N >> K;
  if (K < (N + 1) / 2){
    cout << -1 << endl;
  } else {
    vector<int> a(N), b(N);
    for (int i = 0; i < N; i++){
      a[i] = i;
      b[i] = i;
    }
    int M = N;
    for (int i = 0; i < N - K; i++){
      a.insert(a.begin(), i * 2);
      b.insert(b.begin(), i * 2 + 1);
      M++;
    }
    cout << M << endl;
    for (int i = 0; i < M; i++){
      cout << a[i] + 1 << ' ' << b[i] + 1 << endl;
    }
  }
}