int result = 0;
Dictionary<int, int> set = [];
foreach (int integer in a)
{
	if (!set.ContainsKey(integer))
	{
		set.Add(integer, 1);
	}
	else
	{
		int value = set[integer];
		set[integer] = ++value;
	}
}

if (set.Count == 1)
{
	result = set.Keys.FirstOrDefault();
}
else
{
	result = set.Where(pair => pair.Value == 1).FirstOrDefault().Key;
}

return result;