let currency = prompt('Укажите валюту:').toLowerCase();
let exchangeRate = null;
let exchangeRateSymbol = '';

let ratios = {
	usd: 39.15,
	eur: 39.55
}

switch (currency) {
	case 'usd':
		exchangeRate = ratios.usd;
		exchangeRateSymbol = '$';
		break;
	case 'eur':
		exchangeRate = ratios.eur;
		exchangeRateSymbol = '€';
		break;
	default:
		alert('Неверно указанная валюта');
}

if(currency === 'usd' || currency === 'eur' ) {

	let quantity = +prompt('Укажите величину в гривне для обмена');

	alert(`${quantity} грн в ${currency} = ${(quantity / exchangeRate).toFixed(2)}${exchangeRateSymbol}`);
}