Sumif Multiple Columns With Varying Offset

[Altec23]

I'm trying to figure out how to sum multiple columns from a database given a certain criteria. So, basically I have a database with contract numbers going down column A. In column B through M are the expenses for that particular contract over 12 months. I would like to be able to input the contract number and then the number of months to add together. So for example, if I said 3 months, it would add the first three columns (Jan-Mar). If I said 6 months, it would add the first six columns (Jan-Jun). The database has thousands of contracts that grow with time so it would be ideal to be able to use a format selecting the whole column (i.e. A:A). Any help would be greatly appreciated! By the way, I am using Excel 2007. Thanks!