#include <fstream>

using namespace std;

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

int n,v[27],maxim,nr;
char s[10005];

void rez()
{
	in>>n>>ws;
	in.getline(s,10005);
	for(int i=0;s[i];++i)
	{
		++v[s[i]-'a'+1];
		if(s[i]-'a'+1>maxim)
			maxim=s[i]-'a'+1;
		else
		{
			for(int j=s[i]-'a'+2;j<=maxim;++j)
				nr+=v[j];
		}
	}
	out<<nr;
}

int main()
{
	rez();
	return 0;
}