function isNumberKey(evt)
      {
         var charCode = (evt.which) ? evt.which : event.keyCode
         if (charCode > 31 && (charCode < 48 || charCode > 57))
            return false;

         return true;
      }
function a1()
{
var n = document.form1.n.value;
var a = document.form1.a.value;
var d = document.form1.d.value;
if((n=="" && a=="" && d=="")||(n=="" || a=="" || d==""))
{
alert("please enter value for all the fields");
}
else
{
var e = ((n-1)*d);
var ap = eval(a)+eval(e);
document.form1.ap.value=(eval(a)+eval(e));
var f = 2*a;
var g = (n/2);
var v = eval(f)+eval(e);
document.form1.s.value=g*v;
}
}