#include <stdio.h>
FILE *f=fopen ("litere.in", "r");
FILE *g=fopen ("litere.out", "w");
char x,deumplutura[10];
int n,i,s;
int a[25];

int contor(int k)	{
int i,nr=0;
for (i=k;i<=25;i++)
	nr+=a[i];
return nr;
}

int main() {
fscanf (f, "%d", &n);
fgets (deumplutura, 10, f);
for (i=1;i<=n;i++)
{
	fscanf (f, "%c", &x);
	a[x-97]++;
	s+=contor(x-96);
}
fprintf (g, "%d", s);
return 0;
}