Error in Repeated digital sum in C -

digital sum of 65536 7, because 6+5+5+3+6=25 , 2+5 = 7 want make program above task. didn't correct answer when first addition above 10

program:

#include <stdio.h>  int main() {     int n, sum = 0, temp;     scanf("%d",&n);      while(1)     {         temp = n%10;         sum += temp;         n = n/10;          if(n==0) {             if(sum>=10) {                 n = sum;             }             else {                 break;             }          }     }      printf("%d", sum);     return 0; } 

you can digital sum directly doing, if number non-zero then

value=number % 9 

if value 0 consider sum of digit equal 9.
read link idea how works