Calculating GCD In c

#include<stdio.h>
void main()
{
int n1,n2;
printf("\nEnter two numbers:");
scanf("%d %d",&n1,&n2);
if((n1==0)||(n2==0))

{
printf("GCD NOT POSSIBLE");
getch();
exit();
}
else
{
while(n1!=n2)
{
if(n1>=n2-1)
n1=n1-n2;
else
n2=n2-n1;
}
}
printf("\nGCD=%d",n1);
getch();
}