#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  int sum = 0;
  for (int i = 0; i < n; i++) {
    sum = sum + 4 * (i)*a[i];
  }
  cout << sum;
  return 0;
}
