#include<stdio.h>
long int n,i,j,k,sol,fr[30];
char cuv[10005],*p;
int main()
{
	FILE *f,*g;f=fopen("litere.in","r");g=fopen("litere.out","w");
	fscanf(f,"%ld",&n);
	p=&cuv[0];
	fscanf(f,"%s",p);
	for(i=0;i<n;i++)
	{
		j=(long int)(p[i]-'a'+1);
		fr[j]++;
		for(k=j+1;k<=26;k++)sol+=fr[k];
	}
	fprintf(g,"%ld\n",sol);
	fcloseall();
	return 0;
}