#include <iostream>
#include <fstream>

using namespace std;

ifstream f ("litere.in");
ofstream g ("litere.out");

const int ALPHABET_LENGTH = 26;
const int MAX_WORD_LENGTH = 10000;

int length;
int frequency[ALPHABET_LENGTH];

int required_changes(char c) {
    int changes = 0;
    int alphabet_position = c - 'a';
    for (int i = alphabet_position + 1; i < ALPHABET_LENGTH; i++)
        changes += frequency[i];
    return changes;
}

void solve(){
    char c;
    int result = 0;

    f >> length;
    for (int i = 0; i < length; i++) {
        f >> c;
        frequency[c - 'a']++;
        result += required_changes(c);
    }

    g << result << '\n';
}

int main() {
    solve();
    return 0;
}