class Saifu:
    def __init__(self, yen: int) -> None:
        self.yukichi = yen // 10000
        yen %= 10000
        self.ichiyo = yen // 5000
        yen %= 5000
        self.hideyo = yen // 1000
        yen %= 1000
        self.fh = yen // 500
        yen %= 500
        self.oh = yen // 100


def main():
    C, Y = map(int, input().split())
    fh = Y // 500
    oh = (Y % 500) // 100

    if oh >= C:
        print("no exchange")
        return
    if oh + fh * 5 < C:
        print("can't exchange")
        return

    print(((C + 4) // 5) * 5)


if __name__ == "__main__":
    main()