function calculate() {

// Define them variables

	var current = document.gas.current.value
	var last = document.gas.last.value
	var gallons = document.gas.gallons.value
	var mileage = (current-last)/(gallons);
	var message = 'Current Odometer reading must be greater than Last Odometer reading.  Please try again.'

// Check to make sure current odometer is greater.  If not give error message

	if (document.gas.current.value < document.gas.last.value) {

	alert(message)}

// Round the mileage value.

else {
	if (!isNaN(mileage) &&
		(mileage != Number.POSITIVE_INFINITY) &&
		(mileage != Number.NEGATIVE_INFINITY)) {

		document.gas.mileage.value = round(mileage);
	}


// If they made a mistake, leave result field blank.

	else {

		document.gas.mileage.value = "";
	}
}

function round(x) {
	return Math.round(x*100)/100;
}
}