function Converter() {
  const [km, setKm] = useState(0);

  function handleChange(e) {
    setKm(e.target.value);
  }
  function convert(km) {
    return (km/1.609).toFixed(2);
  }

  return <div>
  <input type="text" value={km}
     onChange={handleChange} />
  <p> {km} km is {convert(km)} miles </p>
  </div>;
}