/**
 *   @FileName	a.cpp
 *   @Author	kanpurin
 *   @Created	2022.06.19 18:11:21
**/

#include "bits/stdc++.h" 
using namespace std; 
typedef long long ll;


int main() {
    int n,k;cin >> n >> k;
    if ((n+1)/2>k) {
        puts("-1");
        return 0;
    }
    cout << k+(n-k)*2 << endl;
    for (int i = 0; i < k; i++) {
        cout << i+1 << " " << i+1 << endl;
    }
    for (int i = k; i < n; i++) {
        cout << i+1 << " " << i-k+1 << endl;
        cout << i-k+1 << " " << i+1 << endl;
    }
    return 0;
}