In this task, find the most appropriate number to replace the blank (indicated with _ ) and express it in words.

Input: Consider Input: Whales have only _ enemies-man and the killer whale.

Output: two


Input: Consider Input: Most people see the world through _ types of visual-receptor cells.

Output: four


Input: Consider Input: Brick chimneys hide a system that circulates air in and out of the building every _ minutes.


Output: six