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

const int MAXN = 1e5;

int n, sumw, w[MAXN + 5];
double gen[MAXN + 5], t[MAXN + 5], p[MAXN + 5];
vector<int> divs[MAXN + 5];

int main() {
	cin >> n;
	for (int i = 1; i <= n; ++i) {
		cin >> w[i];
		sumw += w[i];
	}
	for (int i = 1; i <= n; ++i) {
		gen[i] = (double)w[i] / sumw;
	}
	
	for (int i = 1; i <= n; ++i) {
		for (int j = i; j <= n; j += i) {
			divs[j].push_back(i);
		}
	}
	
	for (int i = 1; i <= n; ++i) {
		double prob = 0;
		for (int j = 0; j < (int)divs[i].size(); ++j) {
			prob += gen[divs[i][j]];
		}
		t[i] = 1.0 / (1.0 - prob);
	}
	
	p[1] = 1;
	
	for (int i = 2; i <= n; ++i) {
		for (int j = 0; j < (int)divs[i].size(); ++j) {
			int d1 = divs[i][j];
			if (d1 == i) continue;
			assert(i % d1 == 0);
			
			int k = lower_bound(divs[i].begin(), divs[i].end(), i / d1) - divs[i].begin();
			for (; k < (int)divs[i].size(); ++k) {
				int d2 = divs[i][k];
				if ((long long)d1 / __gcd(d1, d2) * d2 == i) {
					p[i] += p[d1] * t[d1] * gen[d2];
				}
			}
		}
	}
	
//	for (int i = 1; i <= n; ++i) {
//		cout << t[i] << " ";
//	}
//	cout << endl;
//	for (int i = 1; i <= n; ++i) {
//		cout << p[i] << " ";
//	}
//	cout << endl;
	
	double ans = 0;
	for (int i = 1; i <= n; ++i) {
		ans += p[i] * t[i];
	}
	ans -= 1;
	cout << setiosflags(ios::fixed) << setprecision(10);
	cout << ans << endl;
	return 0;
}