我p5707 WA了最后三个点（70分）

#include<bits/stdc++.h>
using namespace std;
long a,b,s,t,k;
int main()
{
    cin>>a>>b;
    s=a/b+1;
    t=480-s-10;
    s=t%60;
    k=t/60;
    if(s>=10&&k>=10) cout<<k<<":"<<s;
    if(s<10&&k>=10) cout<<k<<":0"<<s;
    if(s<10&&k<10) cout<<"0"<<k<<":0"<<s;
    if(s>=10&&k<10) cout<<"0"<<k<<":"<<s;
    return 0;
}