#include <fstream>

using namespace std;

int fr[300];

int main()
{
    int sol=0,n,i,j;
    char c;
    ifstream fin("litere.in");
    fin>>n;fin.get();
    for(i=1;i<=n;i++)
    {
        fin>>c;
        fr[c]++;
        for(j=c+1;j<='z';j++)
            sol+=fr[j];
    }
    fin.close();
    ofstream fout("litere.out");
    fout<<sol<<"\n";
    fout.close();
    return 0;
}