#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define ALL(v) v.begin(),v.end()
typedef long long ll;
 
#include<bits/stdc++.h>
using namespace std;
 
int main(){
  ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  
  int n,k;
  cin>>n>>k;
  
  int a=(n+1)/2;
  
  if(k<a){
    cout<<-1<<endl;
    return 0;
  }
  
  vector<int> A,B;
  for(int i=1;i<=k;i++){
    A.push_back(i),B.push_back(i);
    for(int j=i+1;j<=n;j++){
      A.push_back(i),B.push_back(j);
      A.push_back(j),B.push_back(i);
    }
  }
  int b=B.size();
  cout<<b<<endl;
  rep(i,b) cout<<A[i]<<" "<<B[i]<<endl;
  
  return 0;
}