417C - Football - CodeForces Solution

#include <bits/stdc++.h>
using namespace std;
int main(){
int n,k;
cin>>n>>k;
if(2*k>=n){
cout<<-1;
return 0;
}
cout<<n*k<<"\n";
for(int i=1;i<=n;i++){
for(int j=i+1;j<=i+k;j++)
cout<<i<<" "<<(j-1)%n+1<<"\n";
}
}