#include<stdio.h>
#include<string.h>
FILE*f=fopen("litere.in","r");
FILE*g=fopen("litere.out","w");
char v[10001];
int fr[125],i,j,n,nr;
int main () {
	fscanf(f,"%d\n",&n);
	for(i=1;i<=n;i++)
		fscanf(f,"%c",&v[i]);
	
	for(i=1;i<=n;i++){
		for(j=v[i]+1;j<='z';j++){
			nr+=fr[j];
		}
		fr[v[i]]++;
	}
	fprintf(g,"%d",nr); 
	fclose(f);
	fclose(g);
	return 0;
}