C# ListBox

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

namespace Programiranje_domaći_1 {
    public partial class Form1 : Form {
        public Form1() {
            InitializeComponent();
        }

        private void dodaj() {
            if(textBox1.Text.Length > 0) {
                listBox1.Items.Add(textBox1.Text);
                textBox1.Text = "";
            }
        }

        private int suma(int[] brojevi, int n) {
            int s = 0;

            for(int i = 0; i < n; i++)
                s += brojevi[i];

            return s;
        }

        private float prosek(int[] brojevi, int n) {
            return (float)suma(brojevi, n) / n;
        }

        private int max(int[] brojevi, int n) {
            int max = brojevi[0];

            for(int i = 0; i < n; i++)
                max = (brojevi[i] > max) ? brojevi[i] : max;

            return max;
        }
        private int min(int[] brojevi, int n) {
            int min = brojevi[0];

            for(int i = 0; i < n; i++)
                min = (brojevi[i] < min) ? brojevi[i] : min;

            return min;
        }

        //Unos dugme
        private void button1_Click(object sender, EventArgs e) {
            dodaj();
        }

        //Klik entera unosi
        private void textBox1_KeyDown(object sender, KeyEventArgs e) {
            if(e.KeyCode == Keys.Enter)
                dodaj();
        }

        //Obrisi izabrano dugme
        private void button2_Click(object sender, EventArgs e) {
            if(listBox1.SelectedIndex != -1)
                listBox1.Items.RemoveAt(listBox1.SelectedIndex);
        }

        //Obrisi sve izabrane dugme
        private void button3_Click(object sender, EventArgs e) {
            int x = listBox1.SelectedItems.Count;

            for(int i = 0; i < x; i++)
                listBox1.Items.Remove(listBox1.SelectedItems[0]);
        }

        //Ocisti dugme
        private void button4_Click(object sender, EventArgs e) {
            listBox1.Items.Clear();
        }

        //Izracunaj dugme
        private void button5_Click(object sender, EventArgs e) {
            int[] izabrani = new int[listBox1.Items.Count];
            int i = 0;

            //Parni radio button
            if(radioButton6.Checked) {
                foreach(string sx in listBox1.Items) {
                    int x = int.Parse(sx);
                    if(x % 2 == 0)
                        izabrani[i++] = x;
                }
            }

            //Neparni radio button
            if(radioButton7.Checked) {
                foreach(string sx in listBox1.Items) {
                    int x = int.Parse(sx);
                    if(x % 2 == 1)
                        izabrani[i++] = x;
                }
            }

            //Svi radio button
            if(radioButton8.Checked) {
                foreach(string sx in listBox1.Items)
                    izabrani[i++] = int.Parse(sx);
            }

            //Izabrani radio button
            if(radioButton9.Checked) {
                foreach(string sx in listBox1.SelectedItems)
                    izabrani[i++] = int.Parse(sx);
            }

            //Suma radio button
            if(radioButton1.Checked) {
                label2.Text = "Rezultat je: " + suma(izabrani, i);
            }

            //Prosek radio button
            if(radioButton2.Checked) {
                label2.Text = "Rezultat je: " + prosek(izabrani, i);
            }

            //Broj radio button
            if(radioButton3.Checked) {
                label2.Text = "Rezultat je: " + i;
            }

            //Maksimalni radio button
            if(radioButton4.Checked) {
                label2.Text = "Rezultat je: " + max(izabrani, i);
            }

            //Minimalni radio button
            if(radioButton5.Checked) {
                label2.Text = "Rezultat je: " + min(izabrani, i);
            }
        }
    }
}