plus x = if x `mod` 5 == 0 then Nothing else Just (x + 1)

f1 n = plus n >>= plus
f2 n = do m <- plus n; plus m
f3 n = do
		m1 <- plus n
		m2 <- plus m1
		plus m2

g3 = getLine >>= putStrLn
g4 = do m <- getLine; putStrLn m