#include <fstream>
using namespace std;
ifstream f("litere.in");
ofstream g("litere.out");
int fr[256],i,n,nr;
char ch,j;
int main()
{
    f>>n;
        for(i=1;i<=n;i++){
            f>>ch;
            for(j=ch+1;j<='z';j++)
                if(fr[j]!=0)
                    nr+=fr[j];
            fr[ch]++;
        }
        g<<nr;
    return 0;
}