#include <bits/stdc++.h>

using namespace std;

int a[110];

int main()

{

    int n,k;

    scanf("%d%d",&n,&k);

    int sum_e = 0,sum_s = 0;

    for (int i = 1;i<=n;i++)

    {

        scanf("%d",&a[i]);

        if (a[i] == 1)

            sum_e++;

        else

            sum_s++;

    }

    int Max = 0;

    for (int i = 1;i<=k;i++)

    {

        int sub_e = 0,sub_s = 0;

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

        {

            if (a[j] == 1)

                sub_e++;

            else

                sub_s++;

        }

        Max = max(Max,abs((sum_e-sub_e)-(sum_s-sub_s)));

    }

    printf("%d\n",Max);

    return 0;

}