Task:

Write a command-line utility computing a probability of getting a value M when throwing N dice.

Functional requirements:

• The program must take two arguments: the number of dice N and the value M.
• The program must output a single floating-point value: the probability of getting M when throwing N dice once.
• The program must exit correctly when given meaningless input, such as negative number of dice etc.

Performance requirements:

• A mean running time over 10 runs with 100 dice must not exceed 2 seconds, with a standard deviation not more than 50 milliseconds.
• A mean running time over 10 runs with 200 dice must not exceed 3.5 seconds, with a standard deviation not more than 75 milliseconds.
• A mean running time over 10 runs with 300 dice must not exceed 7 seconds, with a standard deviation not more than 100 milliseconds.

Implementation requirements:

• Programming language: Ruby.
• Comments are required.
• Unit testing is required.