#include<fstream.h>
#include<string.h>
ifstream f("litere.in");
ofstream g("litere.out");

int main ()
{
    char a[10001],aux;
    long i,j,n,k=0;
    f>>n; f.get();
    f.get(a,n);
    for (i=0; i<strlen(a)-1; i++)
        for (j=i+1; j<strlen(a); j++)
            if (int(a[i])>int(a[j]))
            {
                                    aux=a[i];
                                    a[i]=a[j];
                                    a[j]=aux;
                                    k++;
            }
    g<<k;
    return 0;
}