#include <stdio.h>

int n;
char s[10000];
long g;
void citirea()
{
	scanf("%d\n",&n);
	gets(s);
}

void fatotul()
{
	int i,j;
	for (i=0;i<n;i++)
		for (j=i+1;j<n;j++)
			if (s[i]>s[j])
				g++;
}

void afiseaz()
{
	printf("%ld",g);
}

int main()
{
	freopen("litere.in","r",stdin);
	freopen("litere.out","w",stdout);
	citirea();
	fatotul();
	afiseaz();
	return 0;
}