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