#include <iostream>
#include <fstream>
#include <deque>
#include <stdio.h>

using namespace std;

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

FILE *fin=fopen("date.in","r");
FILE *fout=fopen("date.out","w");

int n,k;

deque<int>a;

void adauga()
{
    int x;
    //fin>>x;
    fscanf(fin,"%d",&x);
    a.push_back(x);

}
int minim()
{
    int minim = a[0];
    for(int i=1;i<a.size();i++)
        if(a[i]<minim)
            minim=a[i];

    return minim;

}
void elimina()
{
    a.pop_front();
}

int main()
{
    fscanf(fin,"%d%d",&n,&k);
    for(int i=1;i<=k;i++)
    {
        int x;
         fscanf(fin,"%d",&x);
        a.push_back(x);

    }

        long long suma=0;

    for(int i=1;i<=n-k+1;i++)
    {
        suma+=minim();
        adauga();
        elimina();
    }
    fprintf(fout,"%lld",suma);
    return 0;
}