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

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    ll n, k;
    cin >> n >> k;
    vector<ll> a(n);
    for(auto &&v : a) cin >> v;
    ll ans = -(1ll << 62), sv = 0;
    for(int i = 0; i < k; i++){
        sv += (i + 1) * a[i];
    }
    ans = max(ans, sv);
    for(int i = k; i < n; i++){
        sv += k * a[i];
        ans = max(ans, sv);
    }
    cout << ans << '\n';
}