#include <stdio.h>
#include <string.h>
int main()
{
//iniatilizere
int i,n;
char a[10000],sa,fa[2];a[0]=NULL;
freopen("litere.in","r",stdin);
freopen("litere.out","w",stdout);
//citire
scanf("%d\n",&n);
for (i=0;i<n;i++)
{
scanf("%c",&sa);
fa[0]=sa;
fa[1]=NULL;
strcat(a,fa);
}

//resolvare
int j;
int interschimbari=0;char aux;
int adev=1;
while (adev)
{
adev=0;
 for (i=0;i<n-1;i++)
 {
  if (a[i]>a[i+1])
  {
   j=i+1;
   interschimbari++;
   adev=1;
   a[i]=a[i]-a[j];
   a[j]=a[i]+a[j];
   a[i]=a[j]-a[i];
  }
 }
}
printf("%d",interschimbari);
return 0;
}