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

const int N = 1e5 + 5;

int s[N], w[N], mu[N];
int n, sum;
long double ans;

int main() {
	scanf("%d", &n);
	mu[1] = 1;
	for (int i = 1; i <= n; i++) {
		for (int j = i * 2; j <= n; j += i) mu[j] -= mu[i];
	}
	for (int i = 1; i <= n; i++) {
		scanf("%d", &w[i]);
		for (int j = i; j <= n; j += i) s[j] += w[i];
		sum += w[i];
	}
	for (int i = 1; i <= n; i++) {
		long double p = (long double)s[i] / sum;
		p = 1 / (1 - p);
		ans += p * mu[n / i];
	}
	printf("%.9Lf\n", ans - 1);
	return 0;
}