#include <fstream>

using namespace std;

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

const int NMAX = 10005;
char sir[NMAX];
int sol;
int l;

void BubbleSort(char *sir){
    bool ok;
    do{
        ok = false;
        for(int i = 0; i < l - 1; ++i){
            if(sir[i] > sir[i + 1]){
                swap(sir[i], sir[i + 1]);
                ok = true;
                sol++;
            }
        }
    }while(ok);
}


int main(){
    in >> l; in.get();
    in >> sir;
    BubbleSort(sir);
    out << sol << "\n";    
    return 0;
}