The Biggest Water Problem

#include <stdio.h>
int sun(int a)
{
    int tmt=a;
    int ptr=0;
    int sum=0;
    while(tmt)
    {
        ptr=tmt%10;
        tmt=tmt/10;
         sum=sum+ptr;
    }
         if(sum>=10)
         {
          return sun(sum);
         }
    
    return sum;
         

}
int main() {
    int a=0;
    scanf("%d",&a);
    int ret=sun(a);
    printf("%d",ret);
    return 0;
}