Let X and Y be random variables with joint probability mass function pXY(x,y) =c(1-2^(-x))^y where X=1,2,3,...,N-1 and Y =1,2,3...

I need to find c.

All I can think of is the double summation Sum(y= 0 to oo) Sum(0 to N-1) (c(1-2^(-x))^y) but this does not seem doable.

I have no ideas what I can try next. Can I please get a hint.

Thanks!!!