# include <cstdio>

using namespace std;

char s;
int cif, i, j, n, ct, ap[28];
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);
        cif = s - 'a';
        ap[cif]++;
        for (j = cif + 1; j <= 25; j++)
            ct += ap[j];
        }
        
    printf("%d",ct);
    
return 0;
}