Language: Ruby

Source code:

def f(n) (2..n).reduce(1, :*) end a = STDIN.readlines.map {|x| x.to_i} puts (a.map {|x| f x}) == [3628800, 265252859812191058636308480000000, 30414093201713378043612608166064768844377641568960512000000000000] ? "Correct" : "Incorrect"

When you run it, it waits for several lines of text (finish with Ctrl-D on Linux or Ctrl-Z on Windows). Flag is correct strings written together.

How to run Ruby:
1. Download Ruby from ruby-lang.org
2. Run in console: ruby task.rb