#include <fstream>

using namespace std;

int sf, inc, n, k, sum = 0, minc;

ifstream cin("deque.in");
ofstream cout("deque.out");

int main()
{
    cin>>n>>k;
    int vec[n];
    for(int i = 1; i <= n; i++)
    {
      cin>>vec[i];
    }
    sf = n;
    while(sf - 2 > inc)
    {
      inc++;
      minc = 1999999999;
      for(int i = 1; i <= k; i++)
      {
        if(minc > vec[inc+i-1])
          minc = vec[inc+i-1];
      }
      sum += minc;
    }

    cout << sum;


    return 0;
}