#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
int n, i;
double suma, arit, a[10001];
suma = 0.00;
arit = 0.00;
cin >> n;
for (i = 1; i <= n; i++)
cin >> a[i];
for (i = 1; i <= n; i++)
suma += 1 / a[i];
arit = n / suma;
cout<<fixed <<setprecision(2)<<arit;
return 0;
}