Given two non negative integers a and b, write a program to find **Greatest common divisor (GCD)** of both the numbers.

**Problem Note**

- GCD of 2 integers a and b is defined as the greatest integer k such that k is a divisor of both a and b.
- Both a and b fit in a 32 bit signed integer.
- Do not use library functions.

**Example 1 **

```
Input: a = 54, b = 24
Output: 6
Explanation: The number 54 can be expressed as a product of two integers in several different ways.
The divisors of 54 are: 1, 2, 3, 6, 9, 18, 27, 54
The divisors of 24 are: 1, 2, 3, 4, 6, 8, 12, 24
The numbers that these two lists share in common are the common divisors of 54 and 24 are: 1, 2, 3, 6
The greatest of these is 6 which is the greatest common divisor of 54 and 24.
```