#include <bits/stdc++.h>
using namespace std;
ifstream fin("deque.in");
ofstream fout("deque.out");

deque<long long> A;
int K, N, x, minim = 1000001, min_sum = 0;

int main()
{
    fin>>N>>K;
    for(int i = 0; i<=N; i++)
    {
        fin>>x;

        if(A.size() == K)
        {
            for(int j = 0; j<K; j++)
            {
                if(A[j] < minim)
                    minim = A[j];
            }
            min_sum += minim;
            // cout<<minim<<" ";
            minim = 1000001;
        }

        A.push_back(x);

        if(A.size() == 4)
            A.pop_front();
    }

    fout<<min_sum;

    return 0;
}