Implement custom section with a Configuration Element Collection c#

I would like to implement a custom configuration section in a project. But something I don't understand so dont work.

I have App.config that looks like this:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <configSections>
    <section name="DepartmentConfigurationSection" type="Statistics.Config.DepartmentSection , Program1"/>
  </configSections>
  <s>
    <Cash>
      <add Number="1" Name="Money" />
    </Cash>
    <Departments>
      <add Id="1" Name="x" />
      <add Id="2" Name="y" />
    </Departments>
  </s>
</configuration>

I create a file called DepartmentSection.cs that cointains the ConfigurationElement,ConfigurationElementCollection and the ConfigurationSection. The class is like this:

 public class DepartmentConfig : ConfigurationElement
    {
        public DepartmentConfig() { }

        public DepartmentConfig(int id, string name)
        {
            Id = id;
            Name = name;
        }

        [ConfigurationProperty("Id", IsRequired = true, IsKey = true)]
        public int Id
        {
            get { return (int)this["Id"]; }
            set { this["Id"] = value; }
        }

        [ConfigurationProperty("Name",  IsRequired = true, IsKey = false)]
        public string Name
        {
            get { return (string)this["Name"]; }
            set { this["Name"] = value; }
        }
    }


    public class DepartmentCollection : ConfigurationElementCollection
    {
        public DepartmentCollection()
        {
            Console.WriteLine("ServiceCollection Constructor");
        }

        public DepartmentConfig this[int index]
        {
            get { return (DepartmentConfig)BaseGet(index); }
            set
            {
                if (BaseGet(index) != null)
                {
                    BaseRemoveAt(index);
                }
                BaseAdd(index, value);
            }
        }

        public void Add(DepartmentConfig depConfig)
        {
            BaseAdd(depConfig);
        }

        public void Clear()
        {
            BaseClear();
        }

        protected override ConfigurationElement CreateNewElement()
        {
            return new DepartmentConfig();
        }

        protected override object GetElementKey(ConfigurationElement element)
        {
            return ((DepartmentConfig)element).Id;
        }

        public void Remove(DepartmentConfig depConfig)
        {
            BaseRemove(depConfig.Id);
        }

        public void RemoveAt(int index)
        {
            BaseRemoveAt(index);
        }

        public void Remove(string name)
        {
            BaseRemove(name);
        }
    }


    public class DepartmentConfigurationSection : ConfigurationSection
    {
        [ConfigurationProperty("Departments", IsDefaultCollection = false)]
        [ConfigurationCollection(typeof(DepartmentCollection),
            AddItemName = "add",
            ClearItemsName = "clear",
            RemoveItemName = "remove")]
        public DepartmentCollection Departments
        {
            get
            {
                return (DepartmentCollection)base["Departments"];
            }
        }
    }

I tried to get the collection from the handler but without success. I tried like this but give me this error: "Unable to initialize the system configuration".

    DepartmentConfigurationSection serviceConfigSection =
    ConfigurationManager.GetSection("s") as DepartmentConfigurationSection;

    DepartmentConfig serviceConfig = serviceConfigSection.Departments[0];