import ast

def limited_eval(node):
    if isinstance(node, ast.Expression):
        return limited_eval(node.body)
    elif isinstance(node, ast.BinOp):
        ops = [
            (ast.Add, lambda a,b: a+b),
            (ast.Sub, lambda a,b: a-b),
            (ast.Mult, lambda a,b: a*b),
            (ast.Div, lambda a,b: a/b)
        ]
        for t, func in ops:
            if isinstance(node.op, t):
                return func(limited_eval(node.left), limited_eval(node.right))
        raise NotImplementedError(f"BinOp with operator {repr(node.op)}")
    elif isinstance(node, ast.Constant):
        return node.value
    else:
        raise NotImplementedError(str(type(node)))

s = "2+2*3+4/4"
x = ast.parse(s, mode="eval")
print(limited_eval(x)) #9.0