#include <fstream>

int N, k = 0, crt[300];


using namespace std;

ifstream cin("litere.in") ;
ofstream cout("litere.out") ;

int main()
{
    cin >> N;
    cin.get();
     char s;
    for(int i = 1 ; i <= N; ++ i)
    {

        cin >> s ;
        ++ crt[s] ;
        for ( int  j = s + 1; j <= 'z' ; ++ j)
            k = k + crt[j] ;


    }
    cout << k ;
    cin.close();
    cout.close();
    return 0;
}