>>775
フィボナッチだってことは解ってる?
https://ja.wikipedia.org/wiki/%E3%83%95%E3%82%A3%E3%83%9C%E3%83%8A%E3%83%83%E3%83%81%E6%95%B0

func(0) = 0
func(1) = 1
func(2) = func(0) + func(1) = 0 + 1 = 1
func(3) = func(1) + func(2) = 1 + 1 = 2
func(4) = func(2) + func(3) = 1 + 2 = 3