#include<stdio.h>
int n,m,f[501];
char s[10001];
int main ()
{
    int i,j;
    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);
    scanf("%d\n",&n);
    gets(s);
    for(i=0;i<n;i++)
    {
        for(j=s[i]-'a'+1;j<='z'-'a';j++)
            m+=f[j];
        f[s[i]-'a']++;
    }
    printf("%d\n",m);
    return 0;
}