样例过了，但测试全WA

#include<bits/stdc++.h>
using namespace std;
int main(){
	int n,m;
	int hour=7,minute=50;
	cin>>n>>m;
	minute-=n/m;
	if(n%m!=0){
		minute--;
	}
	while(minute<0){
		minute+=60;
		hour--; 
		if(hour<0){
			hour=23;
		}
	} 
	if(hour<=9){
		cout<<'0';
	}
	cout<<hour<<":"<<minute;
	return 0;
}