#include <iostream>
#include <fstream>
#include <deque>
using namespace std;
ifstream fin("deque.in");
ofstream fout("deque.out");
int main()
{
    int n,k,cnt=0,pp=1;
    fin>>n>>k;
    deque<long long int>Q;
    long long int s=0;
    for(int i=1; i<=n; i++)
    {
        long long int minim=1000000000;
        if(i==1)
            pp=1;
        else
            pp=2;
        while(cnt<k)
        {
            int x;
            fin>>x;
            i++;
            if(pp==1)
            {
                if(x<minim)
                    minim=x;
            }
            else
            {
                if(Q.front()<=Q.back())
                    minim=Q.front();
                else
                    minim=Q.back();

                if(x<minim)
                    minim=x;
            }
            Q.push_back(x);
            cnt++;
        }
        s=s+minim;
        Q.pop_front();
        cnt--;
        i--;
    }
     fout<<s;
    return 0;
}