#include <fstream>

using namespace std;

const char InFile[]="litere.in";
const char OutFile[]="litere.out";
const int MaxN=10010;

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

char str[MaxN];
int N,nrint,freq[26];

int main()
{
	fin>>N;
	fin>>str;
	fin.close();

	for(register int i=0;i<N;++i)
	{
		int pos=str[i]-'a';
		++freq[pos];
		for(register int j=pos+1;j<26;++j)
		{
			nrint+=freq[j];
		}
	}

	fout<<nrint;
	fout.close();
	return 0;
}