how to deserialize xml with structure of abitrary nested layers of base class?

let say i want to represent arithmetic expression using xml:

<?xml version="1.0" encoding="utf-8" ?><expr><add><num>10</num><num>20</num></add></expr>

and here is my classes to represent this:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Test
{
    public abstract class Evaluable
    {
        public abstract int Eval();
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Xml.Serialization;

namespace Test
{
    [Serializable]
    [XmlRoot("expr")]
    public class Expr
    {
        [XmlElement]
        public Evaluable Evaluable { get; set; }
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Xml.Serialization;

namespace Test
{
    [Serializable]
    [XmlRoot("add")]
    [XmlType("add")]
    public class Add : Evaluable
    {
        [XmlElement]
        public List<Evaluable> Elems { get; set; }

        public override int Eval()
        {
            return Elems.Select(e => e.Eval()).Sum();
        }
    }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Xml.Serialization;

namespace Test
{
    [Serializable]
    [XmlRoot("num")]
    [XmlType("num")]
    public class Num : Evaluable
    {
        [XmlText]
        public int Val { get; set; }

        public override int Eval()
        {
            return Val;
        }
    }
}

but it doesn't work, and it's reasonable, since Serializer doesn't know any thing about the subclass. but how to make it work?

here is my deserialization code:

var serializer = new XmlSerializer(typeof(Expr), new[] { typeof(Add), typeof(Num) });
            Expr expr = null;
            using (var file = new StreamReader("expr.xml"))
            {
                expr = serializer.Deserialize(file) as Expr;
            }