```cpp
#include <iostream>
#include <cstdio>
using namespace std;
int main(){
int s,u,mi=470,hour,minute;
scanf("%d%d",&s,&u);
mi-=s/u;
if(s%u!=0){
mi-=1;
}
hour=mi/60;
minute=mi%60;
if(minute<0){
minute=60+minute;
if(hour<=0){
hour=24+hour;
}
hour--;
}
printf("%.2d:%.2d",hour,minute);
return 0;
}
```
by ZhuZining_king @ 2023-08-16 11:11:46
@[ahheg](/user/978533) 我的AC
by ZhuZining_king @ 2023-08-16 11:12:33
@[ahheg](/user/978533) 记得关注
by ZhuZining_king @ 2023-08-16 11:13:25