import get as nth from Math.Fibonacci;

var input = document.getElementById('n');
var latestKnownValidInput = input.value;
var result = document.getElementById('result');

input.addEventListener('change', function() {
  var newValue = input.value;
  if(isNaN(newValue) || newValue < 0 || newValue.trim().length == 0) {
    input.value = latestKnownValidInput;
  }
  result.textContent = nth(input.value);
});