#include <fstream>

using namespace std;

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

int a[27], i, n, nr;
char s[10000], c;

int main()
{
    fi>>n;
	fi.get();
	fi.get(s, 10000);
	a[s[--n]-'a']++;
	for(i = n-1;i>=0;i--)
	{
	    for(c = s[i]-1;c>='a';c--)
            nr+=a[c-'a'];
        a[s[i]-'a']++;
	}
	fo<<nr;
}