#include <cassert>
#include <cmath>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <functional>
#include <iostream>
#include <limits>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

using namespace std;

using Int = long long;

template <class T1, class T2> ostream &operator<<(ostream &os, const pair<T1, T2> &a) { return os << "(" << a.first << ", " << a.second << ")"; };
template <class T> ostream &operator<<(ostream &os, const vector<T> &as) { const int sz = as.size(); os << "["; for (int i = 0; i < sz; ++i) { if (i >= 256) { os << ", ..."; break; } if (i > 0) { os << ", "; } os << as[i]; } return os << "]"; }
template <class T> void pv(T a, T b) { for (T i = a; i != b; ++i) cerr << *i << " "; cerr << endl; }
template <class T> bool chmin(T &t, const T &f) { if (t > f) { t = f; return true; } return false; }
template <class T> bool chmax(T &t, const T &f) { if (t < f) { t = f; return true; } return false; }
#define COLOR(s) ("\x1b[" s "m")


using Double = double;

int N;
vector<int> W;

int main() {
  for (; ~scanf("%d", &N); ) {
    W.resize(N + 1);
    for (int i = 1; i <= N; ++i) {
      scanf("%d", &W[i]);
    }
    
    Int sumW = 0;
    for (int i = 1; i <= N; ++i) {
      sumW += W[i];
    }
    
    vector<vector<pair<int, int>>> ess(N + 1);
    for (Int x = 1; x <= N; ++x) {
      for (Int y = 1; x * y <= N; ++y) if (__gcd(x, y) == 1) {
        for (Int d = 1; d * x * y <= N; ++d) {
          ess[d * x].emplace_back(d * y, d * x * y);
        }
      }
    }
// cerr<<"ess = "<<ess<<endl;
    
    vector<Double> dp(N + 1);
    for (int u = N; u >= 1; --u) {
      Int s = sumW;
      Double f = 0.0;
      for (const auto &e : ess[u]) {
        if (u < e.second) {
          f += W[e.first] * dp[e.second];
        } else {
          s -= W[e.first];
        }
      }
      f += sumW;
      f /= s;
      dp[u] = f;
    }
    const Double ans = dp[1] - 1.0;
    printf("%.12f\n", ans);
  }
  return 0;
}