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