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

int main()
{
    ifstream f1("deque.in");
    ofstream f2("deque.out");
    long x[500000],n,s,suma,minim;
    long long i,j;
    f1>>n>>s;
    i=0;
    while (f1>>x[i])
        i++;
    suma=0;
    for (i=0; i<=n-s; i++)
    {
        minim=x[i];
        for (j=0;j<s;j++)
        {
            if (minim>x[i+j])
                minim=x[i+j];
        }
        suma+=minim;
    }
    f2<<suma;
    return 0;
}