#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), s1 = 0;
    for(auto &v : x) s1 += (s - v) * v;
    
    cout << fixed << setprecision(15) << (1 + double(s1) / s) << '\n';
}