#include <fstream>
#include <algorithm>
using namespace std;
ifstream fin("litere.in");
ofstream fout("litere.out");
char v[10001],mxm;
int n,i,j,total,poz;
char c;
int main()
{
    fin>>n;
    fin.get();
    fin.getline(v,10000);
    for(i=n-1;i>=0;i--){
           // fout<<"pe poz "<<i<<" vrem sa aducem maximul, momentan avem "<<v[i]<<" \n";
        mxm=v[i];
        for(j=i-1;j>=0;j--)
            if(v[j]>mxm){
            mxm=v[j];
            poz=j;
        }
        if(mxm>v[i]){
            for(int k=poz;k<i;k++){
             swap(v[k],v[k+1]);
             total++;}
        }

    }
    fout<<total;
    return 0;
}