how can multi buttons open navigate pages for android app?

Question

i craete an android application with four buttons in the first interface and i need each button to open another screen when clicked i did it correctly with the first button using this code 

using Android.App;
using Android.Widget;
using Android.OS;

namespace fitness_ghadi
{
    [Activity(Label = "fitness_ghadi", MainLauncher = true , Icon ="@drawable/icon")]
    public class MainActivity : Activity
    {
        protected override void OnCreate (Bundle bundle)
        {
            base.OnCreate(bundle);
            // Set our view from the "main" layout resource
            SetContentView(Resource.Layout.Main);
            Button button = FindViewById<Button>(Resource.Id.second);
            button.Click += delegate {
                StartActivity(typeof(second));
            };
        }

    }
}

and this 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using Android.App;
using Android.Content;
using Android.OS;
using Android.Runtime;
using Android.Views;
using Android.Widget;

namespace fitness_ghadi
{
    [Activity(Label = "second")]
    public class second : Activity
    {
        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);
            SetContentView(Resource.Layout.Second);
            Button button = FindViewById<Button>(Resource.Id.First);
            button.Click += delegate {
                StartActivity(typeof(MainActivity));
            };
            // Create your application here
        }
    }
}

but when i try to make the same code for the rest buttons it did't work i found errors , how can i do it please?