#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <atcoder/modint>

#include <atcoder/segtree>

using namespace std;
using i32 = int32_t;
using u32 = uint32_t;
using i64 = int64_t;
using u64 = uint64_t;
#define rep(i,n) for(int i=0; i<(int)(n); i++)


const i64 INF = 1001001001001001001;
using modint = atcoder::static_modint<1000000007>;


int main(){
    int N; cin >> N;
    int K; cin >> K;
    if(N > K*2){ cout << "-1\n"; return 0; }
    vector<pair<int,int>> ans;
    while(N){
        if(N != K){
            ans.push_back(make_pair(N-1, N-1));
            ans.push_back(make_pair(N-1, N-2));
            ans.push_back(make_pair(N-2, N-1));
            N -= 2;
            K -= 1;
        }
        else{
            ans.push_back(make_pair(N-1, N-1));
            N -= 1;
            K -= 1;
        }
    }
    cout << ans.size() << '\n';
    for(auto [u,v] : ans){
        cout << (u+1) << ' ' << (v+1) << '\n';
    }
    return 0;
}


struct ios_do_not_sync{
    ios_do_not_sync(){
        std::ios::sync_with_stdio(false);
        std::cin.tie(nullptr);
    }
} ios_do_not_sync_instance;