第1631题：gcd

The greatest common divisor of two numbers a and b (not both zero) is the largest number that divides both of them. It is denoted by gcd(a,b). If gcd(a,b)=1, we say that a and b are relatively prime.

Use the Euclidean algorithm to compute the following gcd.

gcd(225,120)=?

Euclidean algorithm： 欧几里得算法、辗转相除法.