#include <stdio.h>
#include <math.h>

int a[10010],b[10010],c[10010],n,rez;
char d[10010];

int main()
{
    freopen("litere.in","rw",stdin);
    freopen("litere.out","wt",stdout);
    scanf("%d\n",&n);
    fgets(d+1,10010,stdin);
    int i,j;
    rez=0;
    for (i=1; i<=n; ++i)
    {
        a[d[i]-'a'+1]++; 
        for (j=d[i]-'a'+2; j<=26; ++j) rez+=a[j];
    }
    printf("%d",rez);
    return 0;
}