#include <iostream>
#include <fstream>

using namespace std;

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

int main()
{int n,k,min1,s,a[5000001],st,dr,i;
    f>>n>>k;
    for(i=1;i<=n;i++)
    {
        f>>a[i];
    }
    st=1; dr=k;
    s=0;
    a[dr+1]=10000001;
    while(dr<=n)
    {
        min1=10000001;
        for(i=st+1;i<=dr;i++)
        {
            if(a[i]<min1)
                min1=a[i];
        }
        if(a[st]<min1)
            s=s+a[st];
        else
            s=s+min1;
        if(a[dr+1]<min1)
            s=s+a[dr+1];
        else
            s=s+min1;
        st=st+2;
        dr=dr+2;
    }
    g<<s;
    return 0;
}