#include <bits/stdc++.h>
using namespace std;
// Pura Gente del Coach Moy
using ld = long double;
signed main() {
  cin.tie(0)->sync_with_stdio(0);
  cout << setprecision(12) << fixed;

  int N;
  cin >> N;
  vector<int> W(N + 1);
  int S = 0;
  for (int i = 1; i <= N; ++i) {
    cin >> W[i];
    S += W[i];
  }

  vector<int> D(N + 1);
  for (int i = 1; i <= N; ++i) {
    for (int j = i; j <= N; j += i) {
      D[j] += W[i];
    }
  }

  vector<ld> E(N + 1);
  for (int i = N; i >= 1; --i) {
    ld C = (ld)D[i] / S, inner = C;
    for (int j = 2 * i; j <= N; j += i) {
      inner += (1 + E[j]) * (D[j] - D[i]) / S;
    }
    E[i] = inner / (1 - C);
  }
  cout << E[1] << '\n';

  return 0;
}