#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("deque.in");
ofstream fout("deque.out");
int v[5000010];
int a[5000010];
int main()
{
    int N, K, i;
    fin>>N>>K;
    long int sum=0, st=1, dr=0;
    for(i=1;i<=N;i++)
    {
    for (i=1;i<=N;i++)
    {
        while (st<=dr && v[i]<=v[a[dr]])
            dr--;
        a[++dr]=i;
        if (a[st] == i-K)
            st++;
        if (i>=K)
            sum += v[a[st]];
    }
    }
    fout<<sum;
    fin.close();
    fout.close();
    return 0;
}