#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;

int N, A[110];

int main() {
  cin >> N;
  for (int i = 1; i <= N - 1; i++) cin >> A[i];
  double ans = 0, res = 1;
  for (int i = 1; i <= N - 1; i++) {
    res *= (double)(1000 - A[i]) / 1000;
  }
  ans = (double)1000 - (double)1000 * res;
  cout << fixed << setprecision(20) << ans << endl;
  return 0;
}