#include<fstream>
#define NMax 10007
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");

int N,Sol;
char S[NMax];
int Frecv[30];

int Numara(int x)
{
    int Nr=0;
    for(int i=x+1;i<=26;++i)
        Nr+=Frecv[i];
    return Nr;
}

int main()
{
    fin>>N>>S;
    for(int i=0;i<N;++i)
    {
        Sol+=Numara(S[i]-'a');
        Frecv[S[i]-'a']++;
    }

    fout<<Sol<<"\n";

    fin.close();
    fout.close();
    return 0;
}