#include<stdio.h>
#include<string.h>
int main ()
{
    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);
    int n,i,pas=0,j;
    scanf("%d",&n);
    char k,a[n];
    gets(a);  
    gets(a);
    for(i=1;i<n;i++)
    for(j=i+1;j<=n;j++)
    if(int(a[i])>int(a[j]))
    {k=a[i];
    a[i]=a[j];
    a[j]=k;
    pas++;
}
  printf("%d ",pas);
    return 0;
}