#include <cstdio>

int n,i,j,v[35],inter;
char s[10005];

int main()
{
    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);
    scanf("%d\n",&n);
    fgets(s,10005,stdin);
    inter=0;
    for (i=0; i<=n-1; i++)
    {
        v[s[i]-'a']++;
        for (j=s[i]-'a'+1; j<='z'-'a'; j++)
            inter=inter+v[j];
    }
    printf("%d\n",inter);
    return 0;
}