New to Java

This is what I am trying to do: https://docs.google.com/fileview?id=0BzufPKSNRUnSOGE5ODI5ZmEtZWE4ZS00ZTZiLTk3ODItOWFjNjk3MmRiOGZl&hl=en.
I'm trying to get the GCD of any two numbers that the user inputs, but when I input two numbers, nothing happens in my program.

I think I messed up in my 'if' statements. Is it allowed to put a "do-while" loop in an if statement? If not, is there another way to find the GCD?
Please help me fix my code. Here is my code:

import java.util.Scanner;

public class GCD
{
public static void main(String[] args)
{
System.out.println("Greatest Common Divisor Finder");
System.out.println();
String choice = "y";
Scanner sc = new Scanner(System.in);
while (choice.equalsIgnoreCase("y"))
{
System.out.print("Enter first number: ");
double first;
first = sc.nextDouble();
System.out.print("Enter second number: ");
double second;
second = sc.nextDouble();
if (first>second)
{
int subtract;
do
{
subtract = (int)first- (int)second;
}
while (first>second);
int gcd;
gcd = (int)second - subtract;
System.out.println("Greatest Common Divisor: " + gcd);
}
else
{
int subtract;
do
{
subtract = (int)second - (int)first;
}
while(second>first);
int gcd;
gcd = (int)first - subtract;
System.out.println("Greatest Common Divisor: " + gcd);
}
System.out.println();
System.out.print("Continue? (y/n): ");
choice = sc.next();
System.out.println();
}
}
}

Thank you so much for the help!