#include <fstream>
using namespace std;
char v[10005];
int frec[10005];


int main()
{
	ifstream fin("litere.in");
	ofstream fout("litere.out");
	int n, i,j ,nr=0;
	char c;
	fin>>n;fin.get();
	
	for(i=1;i<=n;++i)
	{
		c=fin.get();
		v[i]=c;
	}
	
	for(i=1;i<=n;++i)
	{
		for(j=v[i]-'a'+1;j<=26;++j)
			nr=nr+frec[j];
		++frec[v[i]-'a'];
	}
	fout<<nr;
	return 0;
}