#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return true; } return false; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return true; } return false; }
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define mp make_pair
#define si(x) int(x.size())
const int mod=998244353,MAX=300005,INF=1<<30;

vector<int> yaku[MAX];
double dp[MAX];
bool seen[MAX];

int main(){
    
    std::ifstream in("text.txt");
    std::cin.rdbuf(in.rdbuf());
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    ll N;cin>>N;
    vector<ll> PP(N+1);
    vector<double> P(N+1);
    ll Psum=0;
    for(int i=1;i<=N;i++){
        cin>>PP[i];
        Psum+=PP[i];
    }
    for(int i=1;i<=N;i++){
        P[i]=(double)(PP[i])/Psum;
    }
    
    for(ll i=1;i<=N;i++){
        for(ll x=i;x<=N;x+=i) yaku[x].push_back(i);
    }
    
    for(ll i=N;i>=1;i--){
        double sum=0;
        ll sf=0,cn=0;
        for(ll x=i;x<=N;x+=i){
            for(ll a:yaku[x]){
                if(!seen[a]){
                    seen[a]=true;
                    if(x==i){
                        sf+=PP[a];
                    }else{
                        cn+=PP[a];
                        sum+=dp[x]*PP[a];
                    }
                }
            }
        }
        
        dp[i]=(sf+cn+sum)/(Psum-sf);
        for(ll x=i;x<=N;x+=i){
            for(ll a:yaku[x]){
                seen[a]=false;
            }
        }
        
        //cout<<i<<" "<<sf<<" "<<cn<<" "<<dp[i]<<endl;
    }
    
    cout<<fixed<<setprecision(25)<<dp[1]<<endl;
}