#include <fstream>
using namespace std;
ifstream in("litere.in");
ofstream out("litere.out");
int c[26];
int main()
{
    int n;
    char v;
    long long mutari = 0;
    in >> n;
        for(int i = 0; i < n; i++){
                in >> v;
                for(int j = (v - 'a') + 1; j < 26; j++){
                        mutari += c[j];
                }
                c[v - 'a'] += 1;
        }
    out << mutari << "\n";
    return 0;
}