impl Currency {
    fn to_won(&mut self, amount: i32) -> i32 {
        self.exchange_rate = 1200;
        self.exchange_rate * amount
    }
}

fn main() {
    let dollar: Currency = Currency {
        currency_symbol: '$',
        exchange_rate: 1100
    };
    println!("{}", dollar.to_won(200));
}