How to make a function with a diminishing return per iteration?

I'm working on a game, but I can't figure out how to do this. I want to make it so that the curve still increases, so if x = 9999 it will be dramatically higher than if it equals 1, but at the same time, I want each +1 to x to have less and less of a return.
f(x) = y
f(1) = 1
f(2) = 1.9
f(3) = 2.7
and so forth, as an example(Probably doesn't fit a real curve.)