#include <stdio.h>
#include <string.h>
char c;
int v[30];

int parc(int p)
{
    int i,s=0;
    for(i=p+1;i<=26;i++)
        s+=v[i];
    return s;
}

int main()
{
    int poz,n,i,s=0;
    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);
    scanf("%d\n",&n);
    for(i=1;i<=n;i++)
    {
        scanf("%c",&c);
        poz = c-'a'+1;
        v[poz]+=1;
        s+=parc(poz);
    }
    printf("%d",s);
    return 0;
}