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I've tried to search for a simple formula to do this but I can't find any that works for my program.

I found this: (n^2+n)/2=sum, n being the last number in the "series", but it only works if the first number is 1(which mine is necessarly not, since you should be able to input any number).

This is what my program should look like:

Input two numbers : 10 15

Numbers between 10 och 15 : 11 12 13 14

Sum 11 to 14 = 50

Avarage = 12.5