#include<bits/stdc++.h>
#define LL long long
#define db long double
#define pb push_back
#define rep(i,a,b) for(int i=a,i##end=b;i<=i##end;++i)
#define drep(i,a,b) for(int i=a,i##end=b;i>=i##end;--i)
using namespace std;




const int M=1e5+5,inf=1e9;
int n,w[M];
LL sum;
db p[M],E[M],Px[M];
vector<int> G[M];

int gcd(int a,int b){return b?gcd(b,a%b):a;}
void solve(int x,int g){
    int z=x/g;
    rep(i,0,G[z].size()-1){
        int k=G[z][i];
        int u=k*g,v=z/k*g;
        if(u!=x)E[u]+=p[v]*(E[x]+1)/(1-Px[u]);
        if(v!=x)E[v]+=p[u]*(E[x]+1)/(1-Px[v]);
    }
}
void init(){
    rep(z,1,n)rep(k,1,sqrt(z))if(z%k==0 && gcd(k,z/k)==1)G[z].pb(k);
}
int main(){
    scanf("%d",&n);init();
    rep(i,1,n)scanf("%d",&w[i]),sum+=w[i];
    rep(i,1,n)p[i]=(db)w[i]/sum;
    rep(x,1,n){
        rep(i,1,sqrt(x))if(x%i==0){
            Px[x]+=p[i];
            if(x/i!=i)Px[x]+=p[x/i];
        }
        E[x]+=Px[x]/(1-Px[x]);
    }
    drep(i,n,1){
        rep(j,1,sqrt(i))if(i%j==0){
            solve(i,j);
            if(i/j!=j)solve(i,i/j);
        }
        // printf("%d :%.10lf    %.10lf\n",i,Px[i],E[i]);
    }
    printf("%.12Lf\n",E[1]);
    return 0;
}