#include <stdio.h>
#define N 10001
using namespace std;
int n,i,j;
long rez;
char X[N];
int A[30];

FILE *f,*g;

int main()
{
	f=fopen("litere.in","r");
	g=fopen("litere.out","w");
	
	fscanf(f,"%d",&n);
	fgets(X,N,f);
	fgets(X,N,f);
	rez=0;
	for (i='a';i<='z';++i)
		A[i]=0;
	for (i=0;i<n;++i)
	{
		for (j=X[i]+1;j<='z';++j)
			if (A[j]>0)
				rez+=A[j];
		A[X[i]]++;
	}
	fprintf(g,"%ld",rez);
	fclose(f);
	fclose(g);
	return 0;
}