Write a program that asks the user to enter two integers, then calculates and displays their greatest common divisor (GCD).

Hint: The classic algorithm for computing the GCD, known as Euclid's algorithm, goes as follow: let m and n be variables containing the two numbers. If n is 0, then stop: m contains the GCD. Otherwise, compute the remainder when m is divided by n. Copy n into m and copy the remainder into n. Then repeat the process, starting with testing whether n is 0.

Example input:

`12 28`

Example output:

`4`