Cant figure out how to do this!

Im not too good at C++, plus, I am not good at anything. Anyways, I am trying to make a prog that takes a number from the user and then adds all the numbers that lead up to it together. For example

cin>>5 (num)

This is the part I dont know how to do. I know i have to use a loop, and I know how to make it count up to the number, but how do i get it to take the number it gets and put it in some kind of queue that will be added up later (0+1+2+3+4+5)?

cout<<15 (sum)

any help would be great.