#include <iostream>
#include <fstream>
using namespace std;

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

int N,K;
int A[5000001],ST[5000001],DR[5000001];
long long rez;
int main()
{
    fin>>N>>K;
    for(int i=1;i<=N;i++)
        fin>>A[i];
    for(int i=1;i<=N;i++)
        if(i%K==1)
            ST[i]=A[i];
        else
            ST[i]=min(A[i],ST[i-1]);
    DR[N]=A[N];
    for(int i=N-1;i>=1;i--)
        if(i%K==0)
            DR[i]=A[i];
        else
            DR[i]=min(A[i],DR[i+1]);
    int st,dr;
    rez=0LL;
    st=1;
    dr=K;
    while (dr<=N)
    {
        int v;
        v=min(DR[st],ST[dr]);
        rez+=v;
        st++;
        dr++;
    }
    fout<<rez;
    fin.close();
    fout.close();
    return 0;
}