#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
  #define _GLIBCXX_DEBUG
#else
  #define Debug(...) void(0)
#endif
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  int n;
  cin >> n;
  vector<int> a(n - 1);
  rep(i, n - 1) cin >> a[i];
  double ans = 1;
  rep(i, n - 1) ans *= 1 - (double) a[i] / 1000;
  cout << fixed << setprecision(15) << 1000 * (1 - ans) << endl;
  return 0;
}