#include <bits/stdc++.h>
using namespace std;

int main(){

    int n,k;
    cin >> n >> k;

    if(k==0){
        cout << -1 << endl;
    } else {

        cout << k+2*(n-k) << endl;
        for(int i=0;i<k;i++){
            cout << i+1 << " " << i+1 << endl;
        }
        for(int i=k+1;i<=n;i++){
            cout << k << " " << i << endl;
            cout << i << " " << k << endl;
        }
    }

    return 0;
}