I want to insert data from Windows Application of C# to Sql server

When run my code then show this error message "Error :The name 'ConfigurationManager' does not exist in the current context."

How to solve this error?

Code:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Data.SqlClient;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Configuration.Assemblies;
using System.Configuration;




namespace practice
{
    public partial class info : Form
    {
        public info()
        {
            InitializeComponent();
        }

        private void btn_save_Click(object sender, EventArgs e)
        {
            string constr = ConfigurationManager.ConnectionStrings["ConString"].ConnectionString;
            SqlConnection Con =new SqlConnection(constr);
            Con.Open();
            SqlCommand cmd =new SqlCommand();
            cmd.Connection=Con;
            cmd.CommandType=CommandType.Text;
            cmd.CommandText="insert into info values('txt_name.Text+','+txt_mail.Text') ";
            int i=cmd.ExecuteNonQuery();
            if(i>0)
            {
                MessageBox.Show("Data save successfully");
                
            }
            else
            {
                MessageBox.Show("failed");
            }


           
        }

       }
    }