// -fsanitize=undefined,
//#define _GLIBCXX_DEBUG


#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")

#include <iostream>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <queue>
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <random>
#include <stdio.h>
#include <fstream>
#include <functional>
#include <cassert>
#include <unordered_map>
#include <bitset>
#include <chrono>





using namespace std;






#define rep(i,n) for (int i=0;i<n;i+=1)
#define rrep(i,n) for (int i=n-1;i>-1;i--)
#define pb push_back
#define all(x) (x).begin(), (x).end()

#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << " )\n";

template<class T>
using vec = vector<T>;
template<class T>
using vvec = vec<vec<T>>;
template<class T>
using vvvec = vec<vvec<T>>;
using ll = long long;
using pii = pair<int,int>;
using pll = pair<ll,ll>;


template<class T>
bool chmin(T &a, T b){
  if (a>b){
    a = b;
    return true;
  }
  return false;
}

template<class T>
bool chmax(T &a, T b){
  if (a<b){
    a = b;
    return true;
  }
  return false;
}

template<class T>
T sum(vec<T> x){
  T res=0;
  for (auto e:x){
    res += e;
  }
  return res;
}

template<class T>
void printv(vec<T> x){
  for (auto e:x){
    cout<<e<<" ";
  }
  cout<<endl;
}



template<class T,class U>
ostream& operator<<(ostream& os, const pair<T,U>& A){
  os << "(" << A.first <<", " << A.second << ")";
  return os;
}

template<class T>
ostream& operator<<(ostream& os, const set<T>& S){
  os << "set{";
  for (auto a:S){
    os << a;
    auto it = S.find(a);
    it++;
    if (it!=S.end()){
      os << ", ";
    }
  }
  os << "}";
  return os;
}

template<class T>
ostream& operator<<(ostream& os, const tuple<T,T,T>& a){
  auto [x,y,z] = a;
  os << "(" << x << ", " << y << ", " << z << ")";
  return os;
}

template<class T>
ostream& operator<<(ostream& os, const map<int,T>& A){
  os << "map{";
  for (auto e:A){
    os << e.first;
    os << ":";
    os << e.second;
    os << ", ";
  }
  os << "}";
  return os;
}

template<class T>
ostream& operator<<(ostream& os, const vec<T>& A){
  os << "[";
  rep(i,A.size()){
    os << A[i];
    if (i!=A.size()-1){
      os << ", ";
    }
  }
  os << "]" ;
  return os;
}

const int INF = 1e9;

const int M = 2e5+10;


void solve(){
  int N;
  cin>>N;
  vec<int> W(N+1,0);
  rep(i,N) cin>>W[i+1];

  int WS = accumulate(all(W),0);

  vec<int> mue(N+1,1);
  vec<int> prime(N+1,1);
  prime[1] = 0;
  for (int p=2;p<=N;p++){
    if (!prime[p]) continue;
    for (int n=p;n<=N;n+=p){
      mue[n] *= -1;
      prime[n] = 0;
      if ((n % (p*p)) == 0){
        mue[n] = 0;
      }
    }
  }

  vec<int> mue_cum(N+1,0);
  for (int i=1;i<=N;i++){
    mue_cum[i] = mue_cum[i-1] + mue[i];
  }

  vec<int> W_div_sum(N+1,0);
  for (int d=1;d<=N;d++){
    for (int n=d;n<=N;n+=d){
      W_div_sum[n] += W[d];
    }
  }

  long double res = 0;
  for (int n=1;n<=N;n++){
    long double val = (long double) W_div_sum[n] / (long double)(WS - W_div_sum[n]);
    res += val * mue_cum[N/n];
  }

  cout << res << "\n";








}


int main(){

  cout << setprecision(15);

    int T;
    T = 1;
    while (T--){
        solve();
    }

    
    

    
}