#include <fstream>

using namespace std;

ifstream fin("litere.in");
ofstream fout("litere.out");

int n,fr[305];

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