E

#include <bits/stdc++.h>

using namespace std;

int main()
{
    int n;
    cin>>n;

    vector<int> p(n);
    for(auto &e: p)   cin>>e;

    long double p9 = 1;
    vector<long double> pSum(n + 1);

    for (int i = 1; i <= n; i++)
    {
        pSum[i] = (pSum[i - 1] + p9);
        p9 *= 0.9;
    }

    vector<long double> dp(n + 1, -1e18);
    dp[0] = 0;
    for (int i = 0; i < n; i++)
    {
        auto ndp = dp;
        for (int j=0; j<=i; j++)
        {
            if(j+1 <= n)
                ndp[j + 1] = max(ndp[j + 1], dp[j] * 0.9 + p[i]);
        }
        swap(ndp, dp);
    }

    long double ans = -1e18;
    for (int i = 1; i <= n; i++)
    {
        long double x = 1200.0 / sqrt(1.0 * i);
        ans = max(ans, (dp[i] / pSum[i]) - x);
    }

    cout<<fixed<<setprecision(15);
    cout<<ans<<"\n";
}