Error while running ssis package

While running this program to execute ssis package error is coming "The type or namespace name 'Dts' does not exist in the namespace 'Microsoft.SqlServer' (are you missing an assembly reference?)"

using System;

using System.Collections;

using System.Configuration;

using System.Data;

using System.Linq;

using System.Web;

using System.Web.Security;

using System.Web.UI;

using System.Web.UI.HtmlControls;

using System.Net;

using System.Web.UI.WebControls;

using System.Web.UI.WebControls.WebParts;

using System.Xml.Linq;

using System.Data.SqlClient;

using System.IO;

using System.Drawing;

using System.Collections.Generic;

using Microsoft.SqlServer.Dts.Runtime;

using Microsoft.SqlServer.Dts.Runtime.Design;

public partial class update5 : System.Web.UI.Page

{

SqlConnection con = new SqlConnection("Data Source=xxxxxxxx;Initial Catalog=xxxxxx;User ID=xx;Password=xxxxx");

SqlCommand cmd = new SqlCommand();

SqlDataReader dr;

DataTable dt = new DataTable();

protected void Page_Load(object sender, EventArgs e)

{

}

private void Execute_Package()

{

string pkgLocation = @"c:\test.dtsx";

Package pkg;

Application app;

DTSExecResult pkgResults;

Variables vars;

app = new Application();

pkg = app.LoadPackage(pkgLocation, null);

vars = pkg.Variables;

vars["A_Variable"].Value = "Some value";

pkgResults = pkg.Execute(null, vars, null, null, null);

if (pkgResults == DTSExecResult.Success)

Console.WriteLine("Package ran successfully");

else

Console.WriteLine("Package failed");

}

}