#include <fstream>
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
int main() {

    int n;
    fin >> n;
    int freq[26] = {0};
    long cnt=0;
    char ch;
    for (int i = 0; i < n; i++) {
        fin >> ch;
        int c = ch - 'a';
        for (int j = c + 1; j < 26; j++) {
            cnt += freq[j];
        }
        freq[c]++;
    }
    fout << cnt << "\n";
    return 0;
}