#include<stdio.h>
using namespace std;
#define dim 10001
int v[28];

char s;
int main()
{FILE *f=fopen("litere.in","r"), *g=fopen("litere.out","w");
int i,n,m,schimb=0,j,x;
	fscanf(f,"%d ",&n);
	m=n;
for(i=1;i<=n;i++)
	{
	fscanf(f,"%c",&s);
	x=(int)s-96;
	v[x]++;
	for(j=x+1;j<=26;j++)
		if(v[j]!=0)
			schimb+=v[j];
			
	}
	

	
	
				
	fprintf(g,"%d\n",schimb);
	

fclose(f);
fclose(g);

return 0;}