#include <fstream>

using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
int n,sol,ap[50];
char sir[10050];
int main()
{
    fin >> n;
    fin.getline(sir, 100);
    fin.getline(sir, 10005);
    for(int i=0;i<n;i++)
    {
        for(int j=sir[i]-'a'+1;j<='z'-'a';j++)
            sol+=ap[j];
        ap[sir[i]-'a']++;
    }
    fout<<sol<<'\n';
    return 0;
}