n=int(input())
s=input()
print(len(set(s)))

#include <bits/stdc++.h>

using namespace std;

string s, r = "";
long long n;
int main() {
    cin >> n;
    cin >> s;
    sort (s . begin(), s . end());
    s += " ";
    for (long long i = 0; i < n; ++i)
    {
        if (s[i] != s[i + 1]) r += s[i];
    }
    cout << r . size();
    return 0;
}