#include <bits/stdc++.h>
using namespace std;

#define ll long double
#define all(x) x.begin(), x.end()
#define pb push_back
#define f first
#define s second


const int MAXN = 1e5 + 5;
ll dp[MAXN];
vector<int> fac[MAXN];

int main()
{
    ios_base::sync_with_stdio(0); cin.tie(0);
    if (fopen("input.in", "r")) {
        freopen("input.in", "r", stdin);
        freopen("output.out", "w", stdout);
    }
    int n; cin>>n;
    int ar[n+1];
    int tot = 0;
    for(int i=1; i<=n; i++) cin>>ar[i];
    for(int i=1; i<=n; i++){
        for(int j=1; j*j<=i; j++){
            if(i%j == 0) {
                fac[i].pb(j);
                if(i/j != j) fac[i].pb(i/j);
            }
        }
    }

    int denom = 0;
    for(int i=1; i<=n; i++) denom += ar[i];

    ll res = 0;

    for(int i=n; i>=1; i--){


        if(i > 1) dp[i] += 1;

        int stay_same = 0;
        for(int j:fac[i]) stay_same += ar[j];

        // cout<<i<<"|"<<stay_same<<"/"<<denom<<"\n";
        // cout<<i<<"|"<<stay_same<<"<"<<((ll)stay_same * (ll)(denom - stay_same)) / ((ll)stay_same * (ll)stay_same)<<"\n";
        
        dp[i] += (ll)(stay_same) / ((ll)(denom-stay_same));

        for(int j=2; j*i<=n; j++){
            int prob = 0;

            int pw = 1;
            int pwcnt = 0;
            for(int l=1; l<=20; l++) {
                if(i % (pw*j) != 0) break;
                pw *= j;
                pwcnt = l;
            }

            for(int fc:fac[i]){
                if(fc % pw == 0) {
                    int oth = fc * j;
                    prob += ar[oth];
                }
            }
            // cout<<i<<"<<"<<prob<<"|"<<denom<<"\n";
            dp[i] += ((ll) prob / (ll) (denom - stay_same)) * (dp[j*i]);
        }

        // dp[i] += (ll) denom / (ll) (denom - stay_same);
    }
    // cout<<"\n";
    // cout<<"DP: ";
    // for(int i=1; i<=n; i++) cout<<dp[i]<<" ";

    cout<<fixed<<setprecision(20)<<dp[1];


    
}