function calculate()
{  


	var miles = document.getElementById("miles").value;
	if ( miles.length == 0 )
	{
		 alert("Please enter distance to destination in miles (one way).");
		 return;
	}
	miles = fixNumber(miles);
	miles = parseFloat(miles);
	if ( isNaN(miles) )
	{
		 alert("Please enter distance to destination (miles, one way) as a number");
		 return;
	}
	if ( miles <= 0 )
	{
		 alert("The distance is zero, so you won't be spending anything on gas");
		 return;
	}

	var mpg = document.getElementById("mpg").value;
	if ( mpg.length == 0 )
	{
		 alert("Please enter the MPG of your vehicle.");
		 return;
	}
	mpg = fixNumber(mpg);
	mpg = parseFloat(mpg);
	if ( isNaN(mpg) )
	{
		 alert("Please enter the MPG of your vehicle as a number.");
		 return;
	}
	if ( mpg <= 0 )
	{
		alert("Please enter an MPG greater than 0.")
		return;
	}
	
	var cost = document.getElementById("cost").value;
	if ( cost.length == 0 )
	{
		 alert("Please enter the cost of a gallon of gas");
		 return;
	}
	cost = fixNumber(cost);
	cost = parseFloat(cost);
	if ( isNaN(cost) )
	{
		 alert("Please enter the cost of a gallon of gas as a number");
		 return;
	}
	if ( cost <= 0 )
	{
		 alert("If gas is free, the trip won't cost you anything.");
		 return;
	}
	
	var returnTrip = document.getElementById("return_trip").checked;
	if(returnTrip) miles *= 2;
	
	var cost = (miles / mpg ) * cost;
	
	var results = "<p>Your road trip will cost " + formatDollars(cost);
	if(returnTrip)
		results += " round-trip.</p>";
	else
		results += " one way.</p>";
	 
	var x = document.getElementById("results");

	x.innerHTML = results;
	x.style.display = "block";
}