using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static int[][] NArr(long n) => Enumerable.Repeat(0, (int)n).Select(_ => NList).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var p = NList;
        var (c, y) = (p[0], p[1]);
        var c500 = y / 500;
        var c100 = y / 100 % 5;
        var change = (Math.Max(0, c - c100) + 4) / 5;
        if (c500 < change) WriteLine("can't exchange");
        else if (change == 0) WriteLine("no exchange");
        else WriteLine(change * 5 + c100);
    }
}