function clearText1() {
    var a = document.getElementById("textfield1").value;
    if ( isNaN(a)){
      var a = document.getElementById("textfield1").value="";
    }

  }

  function clearText2() {
    var b = document.getElementById("textfield2").value;
    if (isNaN(b)){
      document.getElementById("textfield2").value="";
    }

  }

  function addFun() {
    var num1 = document.getElementById('textfield1').value;
    var num2 = document.getElementById("textfield2").value;
    var firstAnswer = +num1 + +num2;
      document.getElementById("answer1").value= firstAnswer;
  }


  function subFun() {
    var num1 = document.getElementById('textfield1').value;
    var num2 = document.getElementById("textfield2").value;
    var secAnswer = +num1 - +num2;
      document.getElementById("answer2").value= secAnswer;
  }