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

int main(){
    int n;
    cin >> n;
    
    ll s = 0, s1 = 0, v;
    while(n--){
        cin >> v;
        s1 += s * v;
        s += v;
    }
    
    cout << fixed << setprecision(15) << (1 + double(s1 << 1) / s) << '\n';
}