#include <bits/stdc++.h>
#define nmax 5000005
using namespace std;
ifstream fin("deque.in");
ofstream fout("deque.out");
int n,k;
int a[nmax];
int st[nmax];
int dr[nmax];
int main()
{
    fin>>n>>k;
    for(int i=1;i<=n;i++)
    {
        fin>>a[i];
    }
    st[0]=INT_MAX;
    for(int i=1;i<=n;i++)
    {
        if(i%k==1)
        {
            st[i]=a[i];
        }
        else
        {
            st[i]=min(st[i-1],a[i]);
        }
    }
    dr[n]=a[n];
    dr[n+1]=INT_MAX;
    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=0;
    int a=1;
    int b=k;
    while(b<=n)
    {
        rez+=min(dr[a],st[b]);
        a++;
        b++;
    }
    fout<<rez;
    return 0;
}