#include <fstream>
#define Cmax (1<<8)
using namespace std;

int N,Sum,A[Cmax];

int main() {

    char C;
    ifstream in("litere.in");
    ofstream out("litere.out");

    in>>N;
    for(int i=1;i<=N;i++) {
        in>>C;
        Sum+=A[C];
        for(int j=C-1;j>=97;++A[j--]);
        }

    out<<Sum<<'\n';

    in.close();
    out.close();

    return 0;

}