|"What is the sum that I need to put in?"|
"SUM" is a function used to add values. I think you meant to ask:
"What is the function I need to put in?"
"What is the formula I need to put in?"
Excel has a number of "look up" functions that might work for you such as VLOOKUP and MATCH, amongst others. You should review the Help files on them and see which one fits your needs. You can also use Google for more information.
Without knowing how your data is laid out, we can't tell you which function will work best for you. Perhaps if you posted some example data we could help more.
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