how summing the numbers in the listbox

[blankon]

the numbers in the listbox were added from the textbox.
after the numbers added, I want to summing the numbers in the listbox.

I've made this code but It didn't work:

private: System::Void button2_Click(System::Object^  sender, System::EventArgs^  e)
         {
             int i,jum, x, y;             
             {             
             listBox2->Items->Add(txt_add->Text);
             }
 
              x=listBox2->Items->Count;
                        

               for(i=0; i<=x; i++)

                  {

                      y = Convert::ToInt32(listBox2->Items->Item[i]); //ERROR LINE

                   jum += y;

                  }

             txt_total2->Text=jum.ToString();
         }

I got this error:

error C3293: 'Item': use 'default' to access the default property (indexer) for class 'System::Windows::Forms::ListBox::ObjectCollection'

can you tell me what's wrong with that code, and what is the right code to solve my case?