#include<stdio.h>
using namespace std;
const int N = 5000000, INF = -100000000;
int v[N], poz[N];
int main ()
{
    FILE *in, *out;
    in = fopen ("deque.in", "r");
    out = fopen ("deque.out", "w");
    int n, k;
    fscanf (in, "%d%d", &n, &k);
    int i, c = 0;
    v[0] = INF;
    for (i = 1; i <= k; i++)
    {
        fscanf (in, "%d", &v[++c]);
        while (c > 1 && v[c] <= v[c - 1])
                v[--c] = v[c + 1];
        poz[c] = i;
    }
    long long s = (long long) v[1];
    int st = 1;
    for (i = k + 1; i <= n; i++)
    {
        fscanf (in, "%d", &v[++c]);
        while (c > st && v[c] <= v[c - 1])
                v[--c] = v[c + 1];
        poz[c] = i;
        if (i - k + 1 > poz[st])
            st++;
        s += (long long) v[st];
    }
    fprintf (out, "%lld", s);
    return 0;
}