#include <cstdio>
#include <deque>


using namespace std;

struct punct
{
    int x, poz;
};
deque <punct> q;
int main()
{
    int n, k, x, sum=0;
    freopen("deque.in","r",stdin);
    freopen("deque.out","w",stdout);

    scanf("%d %d", &n, &k);
    for (int i = 1; i < k; ++i)
    {
        scanf("%d", &x);

        punct aux;
        aux.x = x;
        aux.poz = i;

        if (!q.empty())
            while ( x < q.back().x )
            {
                q.pop_back();
                if (q.empty()) break;
            }
        q.push_back(aux);
    }

    for (int i = k; i <= n; ++i)
    {
        scanf("%d", &x);

        punct aux;
        aux.x = x;
        aux.poz = i;

        if (!q.empty())
            while ( x < q.back().x )
            {
                q.pop_back();
                if (q.empty()) break;
            }
        q.push_back(aux);
        if (q.front().poz == i-k) q.pop_front();
        sum += q.front().x;
    }

    printf("%d\n", sum);
    return 0;
}