How can I create a formula that tracks total sales after contact was added to a campaign?

Question

I would like to track total sales that are related to people in a campaign only if the sales is after when they were added to the campaign.

Here's what I have but I keep getting an error-

IF(
    ISBLANK(
        SUM(
            IF(
                FirstSaleDate__c >= Contact_ID__c.CreatedDate,
                TotalSales__c, 
                0
            )
        )
    ), 
    0, 
    SUM(
        IF(
            FirstSaleDate__c >= Contact_ID__c.CreatedDate, 
            FirstSaleDate__c, 
            0
        )
    )
)

Here's the error I keep getting-"Error: Field Contact_ID__c does not exist. Check spelling."

This field does exist in my contacts object. Please advise.