#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
const int MAX = 1e4 + 1;
const ll oo = (ll)1e15;
int n, k;
int a[MAX];
ll f[MAX];

#define fi(a,b,c) for (int a=b; a<=c; a++)

int main(){
    //freopen("in.txt", "r", stdin);
    scanf("%d %d", &n, &k);
    fi(i, 1, n) scanf("%d", &a[i]);
    ll ans = oo;
    fi(i, 1, k + 1) {
        ll value = 0;
        int j = i ;
        while (j <= n) {
            value += a[j];
            j += k * 2 + 1;
        }
        j -= k * 2 + 1 ;
        if (j + k < n) continue;
        ans = min(ans, value);
    }

    printf("%lld", ans);

    return 0;
}