#include<iostream>

#include<cstdio>

#include<cstring>

#include<algorithm>

using namespace std;

int sum[],n;

int bigsum[];

int main()

{

    int m;

    while(~scanf("%d%d",&n,&m))

    {

        sum[] = bigsum[] = ;

        for(int i = ;i <= n;i++)

        {

            int nu;

            scanf("%d",&nu);

            sum[i] = sum[i-] + nu;

            bigsum[i] = bigsum[i-] + sum[i];

        }

        long long max = ,num = ;

        for(int i = m;i <= n;i++)

        {

            num = ;

            num = m * sum[i] - (bigsum[i-] - bigsum[i-m-]);

            if(num > max)

            max = num;

        }

        printf("%I64d\n",max);

    }

    return ;

}