#include <bits/stdc++.h>
#include <atcoder/modint>
using namespace std;
using namespace atcoder;
using ll = long long;
using mint = modint1000000007;
#define rep(i,n) for (int i = 0; i < n; i++)
#define rrep(i,n) for (int i = n-1; i >= 0; i--)
#define rep2(i,a,b) for (int i = a; i < b; i++)
#define rrep2(i,a,b) for (int i = a-1; i >= b; i--)
#define rep3(i,a,b,c) for (int i = a; i < b; i+=c)
#define rrep3(i,a,b,c) for (int i = a-1; i >= b; i-=c)
#define all(v) v.begin(),v.end()
#define rall(v) v.rbegin(),v.rend()
template<class T> bool chmax(T &a, T b){if (a < b){a = b;return true;} else return false;}
template<class T> bool chmin(T &a, T b){if (a > b){a = b;return true;} else return false;}

int n,k;
int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cin>>n>>k;
    if(k<=(n-1)/2){
        cout<<-1<<"\n";
        return 0;
    }
    vector<pair<int,int>>Ans;
    int c=0,nl=n;
    while(n>k){
        Ans.push_back(make_pair(c+1,c+2));
        k-=1;
        n-=2;
        c+=2;
    }
    rep(i,nl)Ans.push_back(make_pair(i+1,i+1));
    cout<<Ans.size()<<"\n";
    for(auto[x,y]:Ans)cout<<x<<" "<<y<<"\n";
}