#include <fstream>
#include <cstring>

using namespace std;

ifstream fin("litere.in");
ofstream fout("litere.out");

int freq[30], i, n, j, nr;
char c;

int main() {
    fin >> n;

    for(i = 1;i <= n;i++) {
        fin >> c;

        freq[c - 'a']++;
        for(j = c - 'a' + 1;j <= 26;j++) {
            nr+= freq[j];
        }
    }

    fout << nr << '\n';

    fin.close();
    fout.close();

    return 0;
}