#include <bits/stdc++.h>
using namespace std;
int n, a[102], x = 1, ans;
int main() {
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) scanf("%d", &a[i]);
  for (int i = 2; i <= n; i++) ans += a[i] * 4 * (i - 1);
  printf("%d", ans);
}
