Перегрузка операторов при наследовании

one1.a = one.a + second.a;
one1.b = one.b + second.b;

public class One
{
     public int a;
     public static One operator +(One one, One second)
     {
         One one1 = new One();
         one1.a = one.a + second.a;
         return one1;
     }
 }
public class Two : One
{
      public int b;
      public static Two operator +(Two one, Two second)
      {
          Two one1 = new Two();
 
           one1 = (Two)((One)one + (One)second);// Ошибка
 
           return one1;
      }
 
 
 }

using System;
 
public class One
{
    public int a;
    public static One operator +(One one, One second)
    {
        One one1 = new One();
        one1.a = one.a + second.a;
        return one1;
    }
}
 
public class Two : One
{
    public int b;
    public static Two operator +(Two one, Two second)
    {
        Two one1 = new Two();
 
        one1.a = ((One)one + (One)second).a;
        one1.b = one.b + second.b;
 
        return one1;
    }
 
 
}
 
class Program
{
    static void Main()
    {
        Two q = new Two() { a = 1, b = 10 };
        Two p = new Two() { a = 2, b = 20 };
 
        Two c = q + p;
        Console.WriteLine(c.a + " " + c.b);
 
        Console.ReadKey();
    }
}