# include <stdio.h>
char a[10000];
int k=0,n,i,j;
int main ()
{
freopen ("litere.in","r",stdin);
freopen ("litere.out","w",stdout);
scanf ("%i",&n);
scanf ("%c");
for (i=0;i<=n;i++)
scanf ("%c",&a[i]);
for (i=0;i<n;i++)
{
for (j=0;j<i;j++)
if (a[j]>a[i])
k++;
}
printf ("%i",k);
return 0;
}