pi in Objective C

I keep getting error in my iPhone programing when I try to use pi. I'm trying

    float pNumber = 100*cos(2 * pi * (days/23));

But i get errors that say:

_pi, referenced from

_pi$non_lazy_ptr

I saw somewhere on the internet to use M_PI and it compiles but I don't think it gives me the correct calculation.

When I try:

    float pNumber = 100*cos(2 * M_PI * (15746/23));

I get 0

Thanks