#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];
  }
  vector<int> dp(N, 0);
  for (int i = N - 2; i >= 0; i--){
    if (A[i] == 0){
      dp[i] = 0;
    } else if (A[i + 1] == 1){
      dp[i] = 1;
    } else {
      dp[i] += 2;
      if (i < N - 2){
        dp[i] += dp[i + 2];
      }
    }
  }
  long long ans = 0;
  for (int i = 0; i < N; i++){
    ans += dp[i];
  }
  cout << ans << endl;
}