#include<stdio.h>
long long int a[500005];
long long int sum[500005];
long long int q[500005], l, r;
int main()
{
	long long int n, k;
	scanf("%lld %lld", &n, &k);
	long long int i, j;
	for (i = 0; i < n; i++)
		scanf("%lld", &a[i]);
	sum[0] = 0;
	for (i = 0; i < n; i++)
		sum[i + 1] = sum[i] + a[i];
	long long int m, val = -1e18;
	j = k - 1;
	for (i = k; i <= n; i++)
	{
		if (val < sum[i] - sum[j])
		{
			m = i;
			val = sum[i] - sum[j];
		}
		if (sum[i] < sum[j])
			j = i;
	}
	n = m;
	long long int ans = 0;
	l = r = 0;
	for (j = n - 1, i = k - 1; i > 0; i--)
	{
		for (; j >= i; j--)
		{
			q[r] = j;
			while (l<r && sum[q[r - 1]]>sum[q[r]])
			{
				r--;
				q[r] = q[r + 1];
			}
			r++;
		}
		ans += (sum[n] - sum[q[l]]) * (i + 1);
		n = q[l];
		l++;
	}
	ans += sum[n];
	printf("%lld\n", ans);
	return 0;
}