import java.util.*;
public class Main {
public static void main(String[] args) {
int withdrawalMoney;
double currBalance, remBalance;
Scanner input = new Scanner(System.in);
//System.out.println("CurrentBalance is: ");
currBalance = input.nextDouble();
//System.out.println("Enter the amount to be withdrawal: ");
withdrawalMoney = input.nextInt();
if(withdrawalMoney%5==0 && currBalance>withdrawalMoney+0.50){
remBalance = currBalance - withdrawalMoney - 0.50;
System.out.printf("%.2f",remBalance);
}
else{
System.out.printf("%.2f",currBalance);
}
}
}