#include<bits/stdc++.h>
using namespace std;
const int N=100100;

double sum,ans=-1,f[N];
int n,mu[N],a[N];
int main(){
    ios::sync_with_stdio(0);cin.tie(0);
    cin>>n;
    for(int i=1;i<=n;++i){
        cin>>a[i];
        sum+=a[i];
    }
    mu[1]=1;
    for(int i=1;i<=n;++i){
        f[i]+=a[i];
        for(int j=i+i;j<=n;j+=i){
            mu[j]-=mu[i];
            f[j]+=a[i];
        }
    }
    for(int i=1;i<=n;++i){
        int t=0;
        for(int j=i;j<=n;j+=i)
            t+=mu[j/i];
        ans+=t*sum/(sum-f[i]);
    }
    cout<<fixed<<setprecision(10)<<ans<<endl;
}