#include<fstream.h>
long n,i,j,sir[10005],aux,cate;
char a[10005];
int main(){
	ifstream fin("litere.in");
	ofstream fout("litere.out");
	fin>>n;
	sir[0]=-1;
	for(i=1;i<=n;i++){
		fin>>a[i];
		sir[i]=a[i]-'a';
	}
	for(i=2;i<=n;i++){
		j=i-1;
		aux=sir[i];
		while(aux<sir[j]){
			sir[j+1]=sir[j];
			j--;
			cate++;
		}
		sir[j+1]=aux;
	}
	fout<<cate;
   fin.close();
   fout.close();
	return 0;

}