#include <iostream>
#include <fstream>
#include <algorithm>

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

int n, i, j, nr, rez;
char ch[10005];

int main()
{
    fin >> n;
    for (i = 1; i <= n; i++) {
        fin >> ch[i];
    }

    for (i = 0; i < 26; i++) {

        bool ok = 0;
        nr = 1;
        for (j = 1; j <= n; j++)
            if (ch[j] - 'a' == i) {
                rez += j - nr;
                nr++;
                ok = 1;
            }
        if (ok == 0)    continue;

        nr = 1;
        for (j = 1; j <= n; j++)
            if (ch[j] - 'a' != i) {
                int aux = ch[j];
                ch[j] = ch[nr];
                ch[nr] = aux;
                nr++;
            }
        n = nr - 1;
    }
    fout << rez << "\n";
}