function SoNum() { 
 if((event.keyCode < 48 && event.keyCode != 46) || (event.keyCode > 57)) { 
  event.returnValue = false; 
 } 
}

function subtrai(valor, desc){
 var valor=0, desc=0, result=0;
valor = document.inclui.c33.value;
desc = document.inclui.c36.value;	 
  result = Math.round((valor - parseInt(desc)) *100)/100;
 alert(result);
}