function preventCharacters(allowPeriod, allowNegative)
{
	if (allowPeriod == null) allowPeriod = false
	if (allowNegative == null) allowNegative = false

    var Key = window.event.keyCode

	if (allowPeriod && Key == 46)
		return true;

	if (allowNegative && Key == 45)
		return true;

    if (Key < 48 || Key > 57)
	{
		event.returnValue = false
		event.cancelBubble = true
		return false
	}
	
	return true
}