#define _CRT_SECURE_NO_WARNINGS
#include<iostream>
#include<cstdio>
#include<string>
#include<algorithm>
#include<cmath>
using namespace std;

#define REP(i,n) for(int i=0;(i)<(n);(i)++)

double F(int n) {
	double ans = 0, sum = 0;
	for (int i = 1;i <= n;i++) {
		ans += pow(0.81, i);
		sum += pow(0.9, i);
	}
	ans = sqrt(ans) / sum;
	//cout << "sqrt " << ans << endl;
	return ans;
}

double f(int n) {
	double ans = 0;
	ans = F(n) * 1200;
	return ans;
}

double g(double X) {
	double ans;
	ans = pow(2, X / 800);
	return ans;
}

double g_inv(double R) {
	double ans;
	ans = 800 * log2(R);
	return ans;
}

double Rating(int n, int RPerf[]) {
	double ans = 0, sum = 0;
	for (int i = 1;i <= n;i++) {
		ans += g(RPerf[i-1]) * pow(0.9, i);
		sum += pow(0.9, i);
	}
	//cout << ans << " " << sum << endl;
	ans = ans / sum;
	//cout << ans << endl;
	ans = g_inv(ans) - f(n);
	//cout << ans << endl;
	return ans;
}

int main() {
	int N;
	int RPerf[102];
	cin >> N;
	REP(i, N) {
		cin >> RPerf[i];
	}

	int ans = Rating(N, RPerf);

	//cout << F(N) << endl << f(N) << endl << g(2000) << endl << g_inv(5.66) << endl;

	cout << ans << endl;
	return 0;
}