#include <bits/stdc++.h>

#define rep(i, n, s) for(int (i) = 0; (i) < (n); (i) += (s))

using namespace std;

int main()
{
  long long num, result = 0;
  cin >> num;

  vector<long long> v(num + 5, 0);

  for (int i = 0; i < num; i++)
  {
    long long tmp;
    cin >> tmp;
    v[i] = tmp;
  }

  for (int i = 0; i < num; i++)
  {
    vector<long long>::iterator maxIt = max_element(v.begin(), v.end());
    size_t maxIndex = distance(v.begin(), maxIt);

    result += v[maxIndex];
    if (maxIndex > 0)
      v[maxIndex - 1] = 0;
    v[maxIndex]     = 0;
    v[maxIndex + 1] = 0;
  }

  cout << result << endl;
}