#include <iostream>
#include <fstream>

using namespace std;

ifstream fin("deque.in");
ofstream fout("deque.out");

int n, k;
int a[5000002];
int st[5000002];
int dr[5000002];

int main()
{

    fin >> n >> k;
    for(int i = 1; i <= n; i++)
        fin >> a[i];
    ///se construieste sirul st
    st[0] = 2000000000;
    for(int i = 1; i <= n; i++)
    {
        if(i % k == 1)
        {
            st[i] = a[i];
        }
        else
        {
            st[i] = min(st[i-1], a[i]);
        }
    }
    ///se construieste sirul dr
    dr[n+1] = 2000000000;
    dr[n] = a[n];
    for(int i = n - 1; i >= 1; i--)
    {
        if(i % k == 0)
        {
            dr[i] = a[i];
        }
        else
        {
            dr[i] = min(dr[i+1], a[i]);
        }
    }
    long long rez;
    rez = 0;
    int A, B;
    A = 1;
    B = k;
    while(B <= n)
    {
        rez = rez + min(dr[A], st[B]);
        A++;
        B++;
    }
    fout << rez;
    fin.close();
    fout.close();
    return 0;
}