#include <bits/stdc++.h>
#define int double
using namespace std;
signed main()
{
    int n, s = 0, d = 0; cin >> n;
    vector <int> a(100000);
    for (int i = 0; i < n; i ++)
    {
        cin >> a[i];
        if (a[i] < 0)
        {
            d++;
            s += a[i];
        }
    }
    if (d != 0)
        cout << fixed << setprecision(2) << s/d;
    else
        cout << -1;
}