#include<stdio.h>
int main()
{
float account_balance;
int transaction;
scanf("%d%f",&transaction,&account_balance);
if(transaction<=account_balance && transaction%5==0)
{
account_balance = account_balance - ((float)transaction+0.50);
printf("%.2f",account_balance);
}
else if(transaction%5!=0)
{
printf("%.2f",account_balance);
}
else if(transaction>account_balance){
printf("%.2f",account_balance);
}
return 0;
}