#include <fstream>
using namespace std;

ifstream fin("litere.in");
ofstream fout("litere.out");

int c[1<<5], n, r;
char s[1<<14];


int main()
{
	fin >> n;
	fin.getline(s, 1<<14);
	
	solve();
	fout << r;

	fin.close();
	fout.close();
	return 0;
}

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'];
	}
}