I am receiving a `NaN` error when I run this code. I've been trying to find out why for a while now and can't for the life of me.

To clarify, I am trying to receive;

* the total amount owed by customers
* how many customers are entered

```
function Summarise()
{

    ID = prompt("Enter a Customer ID number ", -1)

    while(ID != -1) 
    {
        var gasUsed = 0
        var total = 0
        gasUsed = prompt("Enter amount of gas used ", "0")
        ///Standard Rate is used to assist in the calculation of customers in excess of 60 Gas Used
        StandardRate = 60 * 1.75

        if(gasUsed <= 60) 
        {
            total= gasUsed * 2
            document.write("Customers ID: " + ID)
            document.write(" has gas usage of " + gasUsed)
            document.write(" and owes $" + total)
            document.write("<br/>")
        } 
        else 
        {
            total= (gasUsed - 60) * 1.50 + StandardRate
            document.write("Customers ID: " + ID)
            document.write(" has gas usage of " + gasUsed)
            document.write(" and owes $" + total)
            document.write("<br/>")

        }

        var totalowed = total + totalowed
        var totalcustomers = ++totalcustomers

        ID = prompt("Enter a Customer ID number ", -1)

    }

    document.write("Total amount owed $" + totalowed)
    document.write("<br/>")
    document.write("Total Number of customers:" + totalcustomers)

}

```