#include <fstream>
#define Nmax 10099
using namespace std;
ifstream f("litere.in");
ofstream g("litere.out");

int N,frec[30],sol;
char S[Nmax];

int main()
{
    f>>N;
    f>>S+1;
    for(int i=1;i<=N;++i)
    {
        ++frec[S[i]-'a'];
        for(int j=S[i]-'a'+1;j<=26;++j)sol+=frec[j];
    }
    g<<sol<<'\n';
    f.close();g.close();
    return 0;
}