#include <fstream>

using namespace std;

const char InFile[]="litere.in";
const char OutFile[]="litere.out";
const int Sigma=26;

ifstream fin(InFile);
ofstream fout(OutFile);

int n,nr,k,v[Sigma+1];
char ch;

int main()
{
	fin>>n;
	for(register int i=0;i<n;++i)
	{
		fin>>ch;
		k=ch-'a';
		for(register int j=Sigma;j>k;--j)
		{
			nr+=v[j];
		}
		++v[k];
	}
	fin.close();
	
	fout<<nr;
	fout.close();
	return 0;
}