evalM :: Exp -> Maybe Int
evalM (Con n) = return n
evalM (Division ex1 ex2) = do
  var1 <- evalM ex1
  var2 <- evalM ex2
  guard $ var2 != 0
  return $ div var1 var2
evalM (Addition ex1 ex2) = do
  var1 <- evalM ex1
  var2 <- evalM ex2
  return $ var1 + var2