#include<fstream>
using namespace std;
long long a[1000009];
int main(){
  int n, m, i;
  ifstream f("scandura.in");
  f>>n>>m;
  m--;
  f>>a[0];
  for(i=1;i<n;i++){
    f>>a[i];
    a[i]+=a[i-1];
  }
  f.close();
  long long cmin=0;
  for(i=n-1;i>0;i-=m)
    cmin+=a[i];
  ofstream g("scandura.out");
  g<<cmin<<'\n';
  g.close();
  return 0;
}