```cpp
#include<bits/stdc++.h>
using namespace std;
int main()
{
double num1,num2,c;
cin>>num1>>num2;
int s=470;
c=ceil(num1/num2*1.0);
s-=c;
if(s<0)s+=1440;
if(s/60<10)cout<<0;
cout<<s/60<<":";
if(s%60<10)cout<<0;
cout<<s%60;
return 0;
}
```
by Leanard @ 2023-10-18 19:32:37