How to make javascript popup window in center of screen for validations.

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C#

ASP.NET

validation

Hello FRIENDS,

am working on asp.net c#

My requirement is i want to make the popup window in center of screen...
Presently it is displaying in the top of screen.

This is the code, that i have wrote for popup.

ScriptManager.RegisterStartupScript(this, this.GetType(), "RunCode", "javascript:alert('Record added Successfully');document.location.href="/KB/answers/Home.aspx";", true);

Please help me, how to display in the center of screen.

Thanks.