#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
typedef long double LD;
const int MAXN = 100005;

int n, w[MAXN], sum;
LD p[MAXN], dp[MAXN];
vector<int> d[MAXN];

void solve() {
    scanf("%d", &n);
    for (int i = 1; i <= n; ++i) {
        scanf("%d", &w[i]);
        sum += w[i];
    }
    for (int i = 1; i <= n; ++i) {
        p[i] = (LD)w[i] / sum;
    }
    for (int i = 1; i <= n; ++i) {
        for (int j = i; j <= n; j += i) {
            d[j].push_back(i);
        }
    }
    for (int i = n; i >= 1; --i) {
        for (int x : d[i]) {
            int a = i / x;
            for (int j = 2; i * j <= n; ++j) {
                if (__gcd(a, j) > 1) continue;
                dp[i] += p[j * x] * (dp[j * i] + 1);
            }
        }
        LD cur = 0;
        for (auto x : d[i]) {
            cur += p[x];
        }
        dp[i] = (dp[i] + cur) / (1.0 - cur);
    }
    printf("%.10Lf\n", dp[1]);
}

int main() {
    solve();
    return 0;
}