#include <bits/stdc++.h>
using namespace std;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    
    int n;
    cin >> n;
    assert(1 <= n && n <= 100000);

    long long s0 = 0, s1 = 0, v;
    for(int i = 0; i < n; i++){
        cin >> v;
        
        assert(1 <= v);
        
        s0 += v;
        s1 += v * v;
    }
    
    assert(s0 <= 1000000000);
    
    cout << fixed << setprecision(15) << (s0 + 1 - (double)(s1) / (s0)) << '\n';
}