codeforces 1100A

#include<bits/stdc++.h>

using namespace std;

/// Typedef
typedef long long ll;
typedef int in;

#define sc1(a) scanf("%lld",&a)
#define sc2(a,b) scanf("%lld %lld",&a,&b)

#define sci1(a) scanf("%d",&a)
#define sci2(a,b) scanf("%d %d",&a,&b)

#define pf1(a) printf("%lld\n",a)
#define pf2(a,b) printf("%lld %lld\n",a,b)

#define pfi1(a) printf("%d\n",a)

#define mx 1000010
#define mod 10000007
#define PI acos(-1.0)

int dr[] = {-2,-2,-1,-1,1,1,2,2};
int dc[] = {-1,1,-2,2,-2,2,-1,1};

int main() {

    ll  tc, num, t;

    //freopen("C:\\Users\\morol\\Desktop\\Clion\\input.txt", "r", stdin);

    sc2(num, tc);
    ll arr[num];

    ll total = 0;
    for( ll i = 0; i < num; i++ ){
        sc1(arr[i]); total += arr[i];
    }

    ll ans = 0;
    for( ll i = 0; i < tc; i++ ){
        ll sum = total;
        for( ll j = i; j < num; j +=tc ) sum -= arr[j];

        ans = max(ans, abs(sum));
    }

    pf1(ans);

    return 0;
}