#include <iostream>
#include <fstream>
#include <deque>

using namespace std;

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

int n,k;

deque<int>a;

void adauga()
{
    int x;
    fin>>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()
{
    fin>>n>>k;
    for(int i=1;i<k;i++)
    {
        int x;
        fin>>x;
        a.push_back(x);

    }

        long long suma=0;

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