#include<stdio.h>
int n,i,k,j;
char s[10001],aux;
int main ()
{
    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);
    scanf("%d\n",&n);
    for(i=1;i<=n;++i)
        scanf("%c",&s[i]);
    for(i=1;i<=n;++i)
        for(j=i+1;j<=n;++j)
            if(s[i]<s[j])
            {
                aux=s[i];
                s[i]=s[j];
                s[j]=aux;
                ++k;
            }
    printf("%d",k);
    return 0;
}