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

class yc2
{
    static void Main()
    {
        int N = int.Parse(Console.ReadLine());
        int cout = 0;
        string str1 = Console.ReadLine();
        
        for(int i=0;i<str1.Length;i++)
        {
            if(str1.Where(c => c == (char)str1[i]).Count() == 1)
            {
                cout++;
            }
        }
        Console.WriteLine(cout);
    }
}