#include <iostream>
#include <algorithm>
#include <iomanip>
#include <vector>
#include <cmath>

using namespace std;

int main()
{
    cout << setprecision(3);
    cout << fixed;
    int n;
    cin >> n;
    vector <double> a;
    int j;
    for (int i = 0; i < n; i++) {
        cin >> j;
        a.push_back(j);
    }
    int left = 0;
    int right = 0;
    int schet = 0;
    for (int i = 0; i < n; i++) {
        if (a[i] == 0) {
            left = i;
            schet++;
            break;
        }
    }
    for (int i = n; i > 0; i--) {
        if (a[i] == 0) {
            right = i;
            schet++;
            break;
        }
    }
    if (schet < 2) {
        cout << "0" << endl;
    }
    double ans = 0;
    for (int i = a[left] + 1; i < a[right]; i++) {
      ans+=a[i];
    }
    cout << ans << endl;
    return 0;
}