#include <fstream>
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
int main()
{
    int n;
    char s[10001];
    int frecventa[26] = {0};
    fin >> n >> s;
    long long nr = 0;
    for(int i = n - 1; i >= 0; i--) {
        int litera = s[i] - 'a';
        for(int j = 0; j < litera; j++) nr += frecventa[j];
        frecventa[litera]++;
    }
    fout << nr;

    return 0;
}