#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <cmath>
#include <tuple>
#include <bitset>

#define rep(i,n) for(i=0; i<n; ++i)
#define in(a) cin >> a
#define out(a,b) cout << a << b
#define print_vec(v) for(auto it=v.begin();it!=v.end();++it)cout<<*it <<" ";cout<<endl
#define print_vec2d(v) for(int i=0;i<v.size();++i){for(int j=0;j<v[i].size();++j)cout<<v[i][j]<<" ";cout<<endl;}cout<<endl
using namespace std;
using lint = long long;

int main(void){
    lint i, j, k, cnt=0;
    lint p, n, m;
    in(n); in(k);
    if(k*2>=n){
        out(k+(n-k)*2,endl);
        rep(i,k)    cout << i+1 << " " << i+1 << endl;
        rep(i,n-k)  cout << i+1 << " " << i+1+k << endl;
        rep(i,n-k)  cout << i+1+k << " " << i+1 << endl;
    }
    else    out(-1,endl);
    return 0;
}