HCF (GCD) of long values

The program must accept two long values X, Y and print their HCF (GCD).

Input Format:
The first line contains X and Y separated by a space.

Output Format:
The first line contains the HCF (GCD) of X and Y.

Boundary Conditions:
1 <=  X, Y <= 999999999999

Example Input/Output 1:
Input:
20 30

Output:
10

Example Input/Output 2:
Input:
999999999999 151515151515

Output:
30303030303

#include <iostream>
using namespace std;
int main(int argc, char** argv)
{
long a,b;
cin>>a>>b;
while(a!=b)
{
    if(a>b)
        a=a-b;
    else
        b=b-a;
}
cout<<a;
}