How to convert QueryString value to Integer in ASP.Net and C#

hi 

this is my code

 protected void Page_Load(object sender, EventArgs e)
    {


        string imageUrl = Request.QueryString["img"];

        int imageHeight = Request.QueryString["h"];
        int imageWidth = Request.QueryString["w"];

        if (imageUrl.IndexOf("/") >= 0 | imageUrl.IndexOf("\\") >= 0)
        {
            //We found a / or \
            Response.End();
        }

        imageUrl = "mypics/" + imageUrl;

        System.Drawing.Image fullSizeImg = null;
        fullSizeImg = System.Drawing.Image.FromFile(Server.MapPath(imageUrl));

        Response.ContentType = "image/jpg";

        if (imageHeight > 0 & imageWidth > 0)
        {
            System.Drawing.Image.GetThumbnailImageAbort dummyCallBack = null;
            dummyCallBack = new System.Drawing.Image.GetThumbnailImageAbort(ThumbnailCallback);

            System.Drawing.Image thumbNailImg = null;


            thumbNailImg = fullSizeImg.GetThumbnailImage(imageWidth, imageHeight, dummyCallBack, IntPtr.Zero);


            fullSizeImg.Save(Response.OutputStream, ImageFormat.Gif);
        }
        else
        {
            fullSizeImg.Save(Response.OutputStream, ImageFormat.Gif);
        }

    }

    public bool ThumbnailCallback()
    {
        return false;

    }

when i run it this error occur

Compilation Error

Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately.  Compiler Error Message: CS0029: Cannot implicitly convert type 'string' to 'int' Source Error:

Line 21:         string imageUrl = Request.QueryString["img"];
Line 22: 
Line 23: int imageHeight = Request.QueryString["h"]; Line 24:         int imageWidth = Request.QueryString["w"];
Line 25: 

i think i should convert strin to int but i dont know how i can do it

thanks