function IsValidFlyBuysNumber(num1,num2,num3,num4,checkbox)
{
    var valid = false;
    if (document.getElementById(checkbox).checked)
    {
        var num1_value = document.getElementById(num1).value;
        var num2_value = document.getElementById(num2).value;
        var num3_value = document.getElementById(num3).value;
        var num4_value = document.getElementById(num4).value;
        
        if (trimText(num1_value) == '' || trimText(num2_value) == '' || trimText(num3_value) == '' || trimText(num4_value) == '')
        {   
            valid = false;
        }
        else if (num1_value.toString().search(/^[0-9]+$/) != 0 || num2_value.toString().search(/^[0-9]+$/) != 0 || num3_value.toString().search(/^[0-9]+$/) != 0 || num4_value.toString().search(/^[0-9]+$/) != 0)
        {
            valid = false;
        }
        else
        {
            valid = true;
        }
        if (!valid)
        {
            alert("You have ticked the 'give me fly buys points' check box. Please fill in a valid fly buys number.");
        }
    }
    else
    {
        valid = true;
    }
    return valid;
}
function trimText(sString)
{
    while (sString.substring(0,1) == ' ')
    {
        sString = sString.substring(1, sString.length);
    }
    while (sString.substring(sString.length-1, sString.length) == ' ')
    {
        sString = sString.substring(0,sString.length-1);
    }
    return sString;
}