#include <stdio.h>

int main()
{
  int num;
  unsigned int sum = 0;

  do {
    printf ("Enter a Non-negative Number: ");
    scanf ("%d", &num);
    if ( num < 0 )
      printf ("Wrong Input!\n");
  } while ( num < 0 );

  while ( 1 ) {
    sum = sum + (num % 10);
    num = num / 10;
    if ( num == 0 ) {
      printf ("Sum of Digits = %u\n", sum);
      if ( sum < 10 ) break;
      num = sum;
      sum = 0;
    }
  }

  return 0;
}