#include <iostream>
#include <fstream>
#define maxn 5000010
using namespace std;

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

long n,k;

int v[maxn], deq[maxn], st, dr;
long long s;
int main()
{
    int i;
    fin>>n>>k;
    st=1;
    dr=0;
    for(i=1;i<=n;i++)
    {
        while(st<=dr && v[i]<v[deq[dr]])
            dr--;
        dr++;
        deq[dr]=i;

        if(deq[st]==i-k)
            st++;

        if(i>=k)
            s=s+v[deq[st]];
    }
    fout<<s;
    fin.close();
    fout.close();
    return 0;
}