#include <bits/stdc++.h>

using namespace std;

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

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

int main() {

    in >> n >> s;
    
    for (int i = 0; i < s.size(); i++) {
        v[s[i] - 'a']++;
        
        for (int j = s[i] - 'a' + 1; j < 26; j++) {
            cnt += v[j];
        }
    }

    out << cnt;

    return 0;
}