fibonacci

fib 0 = 0
fib 1 = 1
fib 2 = 1
fib n = fib (n-1) + fib (n-2)

f a b 0 = 0
f a b 1 = b
f a b n = f b (a+b) (n-1)

fib2 n = f 0 1 n

main = print (fib2 300000)