#include <fstream>

using namespace std;

int c[1<<5],n,r;
char s[10001];
	
void solve ()
{
	for(int i=0;i<n;++i)
	{
        for(int j=s[i]-'a'+1;j<=25;++j)
                r+=c[j];
        ++c[s[i]-'a'];
	}
}

int main ()
{
	
	ifstream fin ("litere.in");
	ofstream fout ("litere.out");
	
	fin>>n;
	fin.getline(s,10001);
	fin.getline(s,10001);
	
	solve ();
	
	fout << r;
	
	fin.close ();
	fout.close();
	return 0;
}