/*
Things to notice:
1. do not calculate useless values
2. do not use similar names
 
Things to check:
1. submit the correct file
2. time (it is log^2 or log)
3. memory
4. prove your naive thoughts 
5. long long
6. corner case like n=0,1,inf or n=m
7. check if there is a mistake in the ds or other tools you use
8. fileio in some oi-contest

9. module on time 
10. the number of a same divisor in a math problem
11. multi-information and queries for dp and ds problems
*/
#include<bits/stdc++.h>
using namespace std;
#define double long double 
#define fi first
#define se second
#define pii pair<int,int>
#define mp make_pair
#define pb push_back
const int mod=998244353;
const int inf=0x3f3f3f3f;
int n;
int w[100005];
double prob[100005],dp[100005];
double cntdiv[100005],del[100005]; 
void solve()
{
	cin>>n;
	for(int i=1;i<=n;i++) cin>>w[i],w[0]+=w[i];
	for(int i=1;i<=n;i++) prob[i]=(double)(w[i])/(double)(w[0]);
	for(int i=1;i<=n;i++) for(int j=i;j<=n;j+=i) cntdiv[j]+=prob[i];
	dp[1]=1;
	for(int i=1;i<=n;i++)
	{
		for(int j=i+i;j<=n;j+=i)
		{
			double coef=cntdiv[j]-del[j]-cntdiv[i];
			for(int l=j;l<=n;l+=j) del[l]+=coef;
			coef/=(1.0-cntdiv[i]);
			dp[j]+=dp[i]*coef;
		}
		for(int j=i;j<=n;j+=i) del[j]=0;
	}
	double res=0.0;
	for(int i=1;i<=n;i++) res+=dp[i]/(1.0-cntdiv[i]);
	cout<<fixed<<setprecision(12)<<res-1.0<<"\n"; 
}
signed main()
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	int _=1;
//	cin>>_;
	while(_--) solve();
	return 0;
}