problematic Haskell code

hello.hs

inputToNum :: String -> Num
inputToNum msg = do
    putStr msg
    i <- getLine
    let o = read i :: Int
    return o

nameDo :: IO ()
nameDo = do
    base <- inputToNum "The base? "
    height <- inputToNum "The height? "

    let area = base * height
    putStrLn ("The area of that triangle is " ++ show area )
    
shinichi@asuka:~/source/hangman ghc hello.hs
[1 of 1] Compiling Main             ( hello.hs, hello.o )

hello.hs:1:25: error:
    • Expecting one more argument to ‘Num’
      Expected a type, but ‘Num’ has kind ‘* -> Constraint’
    • In the type signature: inputToNum :: String -> Num
  |
1 | inputToNum :: String -> Num
  |                         ^^^