#include <fstream>

using namespace std;

int cnt[26];
int n;
string s;

int main() {
    ifstream fin("litere.in");
    ofstream fout("litere.out");
    fin >> n >> s;
    int answer = 0;
    for (int i = 0; i < s.size(); i++) {
        cnt[s[i] - 'a']++;
        for (int j = s[i] - 'a' + 1; j < 26; j++) {
            answer += cnt[j];
        }
    }
    fout << answer;
    return 0;
}