#include <bits/stdc++.h>

#define rep(i, s, n) for (int i = (int)(s); i < (int)(n); i++)
#define rrep(i, s, n) for (int i = (int)(n)-1; i >= (int)(s); i--)
#define all(v) v.begin(), v.end()

using ll = long long;
using ld = long double;
using ull = unsigned long long;

template <typename T> bool chmin(T &a, const T &b) {
    if (a <= b) return false;
    a = b;
    return true;
}
template <typename T> bool chmax(T &a, const T &b) {
    if (a >= b) return false;
    a = b;
    return true;
}

using namespace std;

int main(){
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);

	random_device seed_gen;
	mt19937 engine(seed_gen());
	uniform_int_distribution<int> dist(1, 1000);

	int n; cin >> n;
	
	vector<int> w(n);
	int w_sum = 0;
	rep(i,0,n){
		//w[i] = dist(engine);
		cin >> w[i];
		w_sum += w[i];
	}

	vector<double> p(n+1);
	rep(i,0,n){
		p[i+1] = (double)w[i] / (double)w_sum;
	}

	vector<double> hougan(n+1);
	vector<double> upper(n+1);
	rep(i,1,n+1){
		for (int j=i; j<=n; j+=i){
			hougan[j] += p[i];
			upper[i] += p[j];
		}
	}

	vector<vector<int>> divs(n+1, vector<int>(0));
	vector<vector<int>> divs2(n+1, vector<int>(0));
	rep(i,1,n+1){
		for (int j=2*i; j<=n; j+=i){
			divs[j].push_back(i);
		}
		for (int j=i; j<=n; j+=i){
			divs2[j].push_back(i);
		}
	}

	vector ptr(n+1,vector<int>(0));
	vector<bool> pl(n+1,0);
	rep(i,2,n+1){
		if (pl[i]) continue;
		for (int j=i; j<=n; j+=i){
			pl[j] = 1;
			int cr = 1;
			int x = j;
			while(x%i == 0){
				cr *= i;
				x /= i;
			}
			ptr[j].push_back(cr);
		}

	}

	vector<double> dp(n+1);
	double ans = 0;
	dp[1] = 1;
	rep(i,1,n+1){
		for (int j: divs[i]){
			int tar = 1;
			for (int x: ptr[i]){
				if (j % x != 0){
					tar *= x;
				}
			}
			double var = 0;
			for (int x: divs2[i]){
				if (x % tar == 0){
					var += p[x];
				}
			}
			dp[i] += dp[j] * var / ((double)1 - hougan[j]);
		}

		ans += dp[i] * 1 / ((double)1 - hougan[i]);
	}

	cout << fixed << setprecision(15);
	ans -= 1;
	cout << ans << '\n';

}