#include<iostream>

using namespace std;

void output_edge(int a, int b){
    cout << a << " " << b << "\n";
}

int main(){
    int N,K;
    cin >> N >> K;

    if (N>2*K){
        cout << -1 << endl;
        return 0;
    }

    cout << 2*N-K << endl;

    int p,q;
    for (int i=1; i<=N-K; i++){
        p=2*i-1; q=2*i;
        output_edge(p,p);
        output_edge(p,q);
        output_edge(q,p);
    }

    for (int r=2*(N-K)+1; r<=N; r++){
        output_edge(r,r);
    }
    return 0;
}