#include <iostream>
#include <vector>
#include <cmath>
#include <map>
#include <set>
#include <iomanip>
#include <queue>
#include <algorithm>
#include <numeric>
#include <deque>
#include <complex>
#include <cassert>

using namespace std;

int main(){

    int N;
    long long S=0, S2=0;
    double ans=0, s, s2;
    cin >> N;
    assert(N <= 100000);
    vector<long long> A(N);

    for (int i=0; i<N; i++){
        cin >> A[i];
        S += A[i];
        assert(1 <= A[i]);
    }

    assert(S <= 1e9);

    for (int i=0; i<N; i++){
        S2 += A[i]*A[i];
    }

    s = S;
    s2 = S2;

    cout << fixed << setprecision(18) << (s*s-s2)/s+1 << endl;

    return 0;
}