#include <stdio.h>

char sir[10001], aux;
int used[10001], ap[27];
int n, i, j, k;
int mutari = 0, poz, sum;


int maxim (int p2)
{
	int pozitie = 0, z;
	char Max = 'a';
	for (z=0; z<=p2; ++z)
		if (Max <= sir[z])
		{
			Max = sir[z];
			pozitie = z;
		}
	return pozitie;
}

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;
	}
	//printf ("mutari = %d", mutari);
	
	fprintf (g,"%d", mutari);
	fclose(g);
	
	return 0;
}