#include <fstream>

using namespace std;

ifstream fin("litere.in");
ofstream fout("litere.out");
int f[28];
char a;
int main()
{
    int n,i,j,s=0;
    fin>>n;
    for(i=0;i<n;i++)
    {
        fin>>a;
        f[a-'a']++;
        for(j=a-'a'+1;j<=27;j++)
            s+=f[j];

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