Help with VB 2010 (Numbers)

[Dεlluzion]

Well a friend asked me if I could finish a small program we're supposed to create, using VB 2010.

It's supposed to be a program that calculates costs, along with how much change you'll get, and how much the total is, etc. Like a till.

So I started off and got this error:

Spoiler (Click to View)

Code:

Public Class Form1
    Dim item1 As String
    Dim cost1 As Integer
    Dim item2 As String
    Dim cost2 As Integer
    Dim item3 As String
    Dim cost3 As Integer
    Dim item4 As String
    Dim cost4 As Integer
    Dim totalcost As Integer
    Dim tendered As Integer
    Dim change As Integer

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        item1 = InputBox("Please enter the item name")
        cost1 = InputBox("Please enter the cost of the item")
        item2 = InputBox("Please enter the item name")
        cost2 = InputBox("Please enter the cost of the item")
        item3 = InputBox("Please enter the item name")
        cost3 = InputBox("Please enter the cost of the item")
        item4 = InputBox("Please enter the item name")
        cost4 = InputBox("Please enter the cost of the item")
        tendered = InputBox(" How much has the customer given?")

        totalcost = cost1 + cost2 + cost3 + cost4
        change = tendered - totalcost
        Me.Show()

Conversion from string "" to type 'Integer' is not valid. [COST1]

Anybody know what I have done wrong here?

[Coding Support]

Your trying to convert a string("") to an integer, and you cant.