#include <fstream>
#define CH sir[i]-'a'
#define nMax 10001
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
char sir[nMax];
int n, i, j, Rez, viz[nMax];
int main()
{
    fin>>n>>sir+1;
    for(i=1;i<=n;i++)
    {
        for(j=CH+1;j<26;j++)
            Rez+=viz[j];
        viz[CH]++;
    }
    fout<<Rez;
    return 0;
}