#include<stdio.h>
int a[27];
int main()
{
    
 freopen("litere.in","r",stdin);
 freopen("litere.out","w",stdout);
 
 int n,val,sol=0,i;
 char ch;   
    scanf("%d",&n);
    scanf("%c",&ch);
    
    do{
    
    val=int(ch)-96;
    
    sol+=a[val];
    
    for(i=val-1; i>=1; --i)
    ++a[i];
    
    }while( scanf("%c",&ch)!=EOF );
    
    printf("%d\n",sol);
    
    return 0;
}