#include<stdio.h>
#include<string.h>

#define N 10000

void inter(char &c1, char &c2)
{
	char aux;
	aux=c1;
	c1=c2;
	c2=aux;
}

int main()
{
	freopen("litere.in","r",stdin);
	freopen("litere.out","w",stdout);
	
	char s[N];
	int i,j,cnt=0;
	
	gets(s); gets(s);
	for(i=0; i<strlen(s)-1; i++)
	for(j=i+1; j<strlen(s); j++) 
		if(s[i]>s[j]) ++cnt;
	
	printf("%d\n",cnt);
	
	return 0;
}