#include <stdio.h>

short int v[10010],alf[30];
int i,n,j,cnt;

int main ()
{
    freopen ("litere.in","r",stdin);
    freopen ("litere.out","w",stdout);
    
    scanf ("%d\n",&n);
    for (i=1;i<=n;i++){
        char x;
        scanf ("%c",&x);
        v[i]=x-96;
        }
        
    for (i=1;i<=n;i++){
        alf[v[i]]++;
        for (j=v[i]+1;j<=26;j++) cnt+=alf[j];
        } 
        
    printf ("%d",cnt);

    return 0;
}