#include <bits/stdc++.h>

using namespace std;

int a[5000000], s, d;

int main()
{
    int n, k;
    int64_t suma=0;

    ifstream f("deque.in");
    ofstream g("deque.out");

    f>>n>>k;
    for(int i=0;i<n;++i)
    {
        int x;
        f>>x;
        a[i]=x;
    }

    s=k-1;
    d=n-1;

    while(s<=d)
    {
        int x=a[s], y=a[d];
        if(s==d)
        {
            for(int i=1;i<k;++i)
                if(a[s-i]<=x)
                    x=a[s-i];
            suma+=x;
        }
        else
        {
            for(int i=1;i<k;++i)
            {
                if(a[s-i]<=x)
                    x=a[s-i];
                if(a[d-i]<=y)
                    y=a[d-i];
            }
            suma+=y;
            suma+=x;
        }

        s++;
        d--;
    }


    g<<suma;
    return 0;
}