function calcula(){ 


	
    var operando1 = document.calc.ti.value;
    var operando2 = document.calc.te.value;
    var result = eval(operando1 - operando2);
    document.calc.dif.value = result; 
	
	var gas = 0.041198;
	var m2 = document.calc.superficie.value;
	
	
	var decimales = 4;
decimales = (!decimales ? 2 : decimales);
	
	
	
	var h2=720;
	
	var tot1=eval(2.4*gas*m2*h2*result);
	var tot2=eval(1.4*gas*m2*h2*result);
	
	var tot= eval(tot1 - tot2);
	
	
	document.calc.total.value = (Math.round(tot * Math.pow(10, decimales)) / Math.pow(10, decimales))/(1000);
	document.calc.total.value = (Math.round(document.calc.total.value * Math.pow(10, decimales)) / Math.pow(10, decimales));	
	
}