using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        List<int> input = new List<int> {

            1,1,2,3,2,2,3,4,5,5,6,7

        };

        List<int> result = AppearsOnce(input);

        for(int i = 0; i < result.Count; i++) {

            Console.WriteLine(result[i]);

        }

    }

    public static List<int> AppearsOnce(List<int> input) {

        List<int> result = new List<int>();

        List<int> duplicates = new List<int>();

        for(int i = 0; i < input.Count; i++) {

            if(duplicates.Contains(input[i])) continue;

            if(result.Contains(input[i])) {

                result.Remove(input[i]);

                duplicates.Add(input[i]);

                continue;

            }

            result.Add(input[i]);

        }

        return result;

    }

}