15-1

string sentence = Console.ReadLine();
List<char> chars = new List<char>();
List<int> counts = new List<int>();

foreach (char c in sentence)
{
    if (!chars.Contains(c))
    {
        chars.Add(c);
        counts.Add(1);
    }
    else
    {
        int index = chars.IndexOf(c);
        counts[index]++;
    }
}

for (int i = 0; i < chars.Count; i++)
{
    Console.WriteLine($"[{chars[i]}] - {counts[i]}");
}