#include <stdio.h>
int i,j,k,n,nr;
char a[10001];
int alf[27];
    
int main()
{
    
    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);
    
    scanf("%d",&n);scanf("%c",&a[1]);
    for (i=1; i<=n; i++)
        scanf("%c",&a[i]);
    
    for (i=1; i<=n; i++)
    {
        alf[a[i]-96]++;
		for (j=a[i]-96+1; j<=26; j++)
			nr+=alf[j];
    }     
    
    printf("%d\n",nr);

    return 0;    
}