#include <fstream>
using namespace std;
int sol,n,i,j,F[150];
char V[10010];
int main ()
{
    ifstream fin ("litere.in");
    ofstream fout("litere.out");
    fin>>n;
    for(i=1;i<=n;i++)
        fin>>V[i];
    for(i=1;i<=n;i++)
    {
        for(j=V[i]+1;j<='z';j++)
            sol=sol+F[j];
        F[V[i]]++;
    }
    fout<<sol;
    return 0;
}