#include <stdio.h>

char sir[10001];
int ap[27];
int n, i, j;
int mutari, sum;

int main()
{
	FILE *f = fopen ("litere.in","r");
	FILE *g = fopen ("litere.out","w");
	fscanf (f,"%d", &n);
	fscanf (f,"%s", sir);
	fclose(f);

	for (i=0; i<n; ++i)
	{
		ap[sir[i] - 'a'] ++;
		sum = 0;
		for (j=sir[i]-'a'+1; j<=26; ++j)
			if (ap[j] == 0)
				continue;
			else
				sum += ap[j];
		mutari += sum;
	}

	fprintf (g,"%d", mutari);
	fclose(g);

	return 0;
}