#include<cstdio>
#include<string.h>
int ns,i,j,s2;
char s[10010],f[10010];
int main()
{
	freopen("litere.in","r",stdin);
	freopen("litere.out","w",stdout);
	scanf("%d\n",&ns);
	gets(s);
	for(i=0;i<ns;i++)
	{
		for(j=0;j<i;j++)
			if(s[i]<s[j])
				f[i]++;
	}
	s2=0;
	for(i=0;i<ns;i++)
		s2+=f[i];
	printf("%d",s2);
	return 0;
}