#include <fstream>
#include <algorithm>

using namespace std;

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

int n, k, f[30];
char s[10001];

int main()
{
    fin >> n;

    for(int i = 0; i < n; i++)
    {
        fin >> s[i];

        f[s[i] - 'a']++;

        for(int j = s[i] - 'a' + 1; j < 26; j++)
            k += f[j];
    }

    fout << k;

    fin.close(); fout.close();
    return 0;
}