/* -*- coding: utf-8 -*-
 *
 * 1993.cc:  No.1993 Horse Racing - yukicoder
 */

#include<cstdio>
#include<algorithm>
 
using namespace std;

/* constant */

const int MAX_N = 100;
const int L = 1000;

/* typedef */

/* global variables */

int as[MAX_N];

/* subroutines */

/* main */

int main() {
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; i++) scanf("%d", as + i);

  double x = L;
  for (int i = 0; i < n; i++) x *= (double)(L - as[i]) / L;

  printf("%.12lf\n", L - x);

  return 0;
}