#include<stdio.h>
FILE *f=fopen("litere.in","r"), *g=fopen("litere.out","w");
int i,j,n, ver[10010];
long s;
char v[10010],m;
int main()
{
	fscanf(f,"%d%c", &n, &m);
	for(int i=1;i<=n;i++)
		fscanf(f,"%c", &v[i]);
	for(int i=1;i<=n;i++)
	{
		ver[v[i]-'a']++;
		for(j=v[i]-'a'+1; j<='z'-'a'; j++)
			s+=ver[j];
	}
	fprintf(g,"%ld",s);
	return 0;
}