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

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n;
    cin >> n;
    
    vector<ll> x(n);
    for(auto &v : x) cin >> v;
    ll s = accumulate(x.begin(), x.end(), 0ll);
    ll s1 = accumulate(x.begin(), x.end(), 0ll, [&](ll lhs, ll rhs){return lhs + rhs * (s - rhs);});
    
    cout << fixed << setprecision(15) << (1 + double(s1) / s) << '\n';
}