FIND OUT GENERIC ROOT OF A NUMBER USING C PROGRAM

void main()

{

    long int num,sum,r;

    clrscr();

    printf("\nEnter a number:-");

    scanf("%ld",&num);

    while(num>10)

    {

         sum=0;

         while(num)

         {

             r=num%10;

             num=num/10;

             sum+=r;

         }

         if(sum>10)

             num=sum;

         else

             break;

    }

    printf("\nSum of the digits in single digit is: %ld",sum);

    getch();

}