# include <fstream>
using namespace std;
int n, c[30], sol;
char s[10003];

void read ()
{
	ifstream fin ("litere.in");
	fin>>n;
	fin.get();
	fin.getline(s, 10001);
}

void solve ()
{
	for (int i=0;i<n;i++)
	{
		for (int j=(int)(s[i]-'a'+1);j<=(int)('z'-'a');j++)
			sol+=c[j];
		c[s[i]-'a']++;
	}
}

int main ()
{
	read ();
	solve ();
	ofstream fout ("litere.out");
	fout<<sol;
	return 0;
}