c语言 acm提交显示time limit exceed是什么原因

#include<stdio.h>
int main(){
int b,c,m=0;unsigned long long a;
while(~scanf("%d %d",&a,&b)){
if(b==9){
while(a>0){
m=m+a%10;
a=a/10;}
c=m;
while(c>10){
while(c>0){
m=m+c%10;
c=c/10;}
c=m;
}
printf("%d\n",c);
}
else if(b==3){
while(a>0){
m=m+a%10;
a=a/10;
}
c=m;
printf("%d\n",c);
}
else printf("%d\n",a%b);
}
return 0;
}

超时