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()