[ruslan_mart]

<form name="game">
  <input id="number" type="text">
  <input type="submit" value="OK">
</form>

var form = document.forms.game;
var number = 5;

form.onsubmit = function() {
	
  var value = form.elements.number.value;
  
  if(value === '' || isNaN(+value)) {
  	alert('Введите число!');
  }
  else if(+value === number) {
  	alert('Вы угадали!');
  }
  else {
  	alert(+value < number ? 'Больше' : 'Меньше');
  }

  return false;
};