Sum of a number indeterminated of objects

Hi everybody,

I'm trying to write down a particular type of function, but I don't really know how to do it with the knowledge that I have :

I have a box (in this case I39) which countains a number that I insert and relied to a function sum :

sum from 1 to I39(the value inside the box) of all the elements H(25+i). Does anybody know how to help me please ?