#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  vector<long double> a(n - 1);
  for (int i = 0; i < n - 1; i++) {
    cin >> a[i];
  }
  long double sum = 1000.0;
  for (int i = 0; i < n - 1; i++) {
    sum *= ((1000.0 - a[i]) / 1000.0);
  }
  cout << fixed << setprecision(15);
  cout << 1000.0 - sum << endl;
  return 0;
}