1. #include<stdio.h>
  2. int main()
  3. {
  4.     int n = 0;
  5.     int sum = 0;
  6.     scanf("%d", &n);        
  7.     do
  8.     {
  9.         sum = 0;
  10.         while (n)
  11.         {
  12.             sum += n % 10;
  13.             n /= 10;
  14.         }
  15.         n = sum;
  16.     } while (sum > 9);
  17.     printf("%d", sum);
  18.     return 0;
  19. }