#include <cstdio>
#include <vector>

#define PB push_back

using namespace std;

typedef long double ld;

const int N = 1e5 + 500;

vector < int > v[N], fact[N];
int W[N], n;
ld dp[N];

int lower_limit(int t, int z) {
	int ret = 1;
	for(int p : fact[t]) {
		while(!(z % p)) z /= p, ret *= p;
	}
	return ret;
}

int main() {
	scanf("%d", &n);
	int sve = 0;
	for(int i = 1;i <= n;i++) {
		scanf("%d", W + i);
		sve += W[i];
	}
	for(int i = 1;i <= n;i++) {
		if(i > 1 && fact[i].empty()) {
			for(int j = i;j <= n;j += i)
				fact[j].PB(i);
		}
		v[i].resize(n / i + 1);
		for(int j = 1;i * j <= n;j++)
			for(int k = 1;i * k * j <= n;k++)
				v[i][k * j] += W[i * j];
	}
	for(int x = n;x >= 1;x--) {
		for(int z = x;z <= n;z += x) {
			int a = lower_limit(z / x, z);
			dp[x] += (1 + dp[z]) * v[a][z / a];
		}
		dp[x] /= (sve - v[1][x]);		
	}
	printf("%.10Lf\n", dp[1]);
	return 0;
}