#include <fstream>
using namespace std;
ifstream fin ("litere.in");
ofstream fout ("litere.out");
int fr[256],n,nr,i;
char s[10001],j;
int main()
{
    fin>>n;
    fin>>s+1;
    for (i=1; i<=n; i++)
    {
        fr[s[i]]++;
        for (j=s[i]+1; j<='z'; j++)
            nr=nr+fr[j];
    }
    fout<<nr;
    return 0;
}