#include <algorithm>
#include <cmath>
#include <cstdio>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
using namespace std;
using ll = long long;
int main() {
  int n;
  cin >> n;
  vector<double> a(n);
  for(int i = 0; i < n - 1; i++) {
    cin >> a[i];
  }
  double ans = 1000.0;
  double mul = 1000.0;
  for(int i = 0; i < n - 1; i++) {
    mul *= (1000 - a[i]) / 1000.0;
  }
  printf("%.10f\n", ans - mul);
  return 0;
}