add a listener to a changed value

Hi,
i'm creating a simple app who get 6 numbers (each number represent human) from 6 different txt files. the txt files are changing along the day for several times, so the column for the 6 people updates in every change. this happend du to a timer loop, who checks in every second for a change, in all of the 6 files.
now, i need a column to sum the month changes. the problem: because it's in a loop of a timer, i can't check if there is a change in the value, and i get sum for every second.

please help me. i want to check if there is a change between the values in the 6 files, and if so, only then it will sum to the month conclusion.

thanks!!