#include <cstdio>

using namespace std;
int i,aux,n,b,k,j,p,a[2500],m,s,ax,cont;
char sir[10000];

int main()
{
    freopen ("litere.in","r",stdin);
    freopen ("litere.out","w",stdout);

    scanf("%d\n",&n);
    gets(sir);
    for(i=0;i<n;i++)
    {
    for(j=i+1; j<n; j++ )
    if(sir[j]<sir[i])
    {
        ax=sir[j];
        sir[j]=sir[i];
        sir[i]=ax;
        cont++;
    }
    }
    printf("%d",cont);

}