#include <algorithm>
#include <fstream>

using namespace std;

typedef char cha08;
typedef int int32;

const cha08 Input[] = "litere.in";
const cha08 Output[] = "litere.out";

const int32 Alf = 26;
const int32 Dim = 10001;

cha08 s[Dim];
int32 N, XXX;
int32 f[Alf];

int32 Que( int32 x ) {

    int32 i, cnt;

    for( i = x + 1, cnt = 0; i < Alf; ++i )
        cnt += f[i];

    return cnt;
}

int32 main() {

    ifstream fin( Input );
    ofstream fout( Output );

    int32 i;

    fin >> N;
    fin.ignore( 1, '\n' );
    fin.getline( s, Dim );

    for( i = 0; i < N; ++i ) {

        XXX += Que( s[i] - 'a' );
        ++f[s[i] - 'a'];
    }

    fout << XXX;

    fin.close();
    fout.close();

    return 0;
}