Problem:
Greatest Common Divisor

Write a program which finds the greatest common divisor of two natural numbers a and b

Input

a and b are given in a line sparated by a single space.

Output

Output the greatest common divisor of a and b.

Constrants

1 ≤ a, b ≤ 109

Hint

You can use the following observation:

For integers x and y, if x ≥ y, then gcd(x, y) = gcd(y, x%y)

Sample Input 1

54 20

Sample Output 1

2

Sample Input 2

147 105

Sample Output 2

21

Solution in Ruby (judge verdict: Accepted)

a,b=gets.split
x=a.to_i
y=b.to_i
if x<y
  tmp=x
  x=y
  y=tmp
end
if x.between?(1,1000000000)==false then
   puts"Please input [1<= a,b <=1000000000]"
   exit
end

if y.between?(1,1000000000)==false then
   puts"Please input [1<= a,b <=1000000000]"
   exit
end

r=x%y

while r!=0 do
  x=y
  y=r
  r=x%y
end

puts y