CS0246: The type or namespace name StringBuilder could not be found

Hi 

i use this code in my page

    protected void Button1_Click(object sender, EventArgs e)
    {
        string url = "page2.aspx";
        StringBuilder sb = new StringBuilder();
        sb.Append("<script type = 'text/javascript'>");
        sb.Append("window.open('");
        sb.Append(url);
        sb.Append("');");
        sb.Append("</script>");
        ClientScript.RegisterStartupScript(this.GetType(),
                "script", sb.ToString());
    }

but this error occur

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: CS0246: The type or namespace name 'StringBuilder' could not be found (are you missing a using directive or an assembly reference?)

Source Error:

Line 145:    {
Line 146:        string url = "page2.aspx";

Line 148:        sb.Append("<script type = 'text/javascript'>");
Line 149:        sb.Append("window.open('");

how i can solve it?

Thanks