#include <iostream>
#include <fstream>
using namespace std;

ifstream f ("litere.in");
ofstream g ("litere.out");

int l[27];

int MaiMariDecat (char a)
{
    int nr=0;
    for (int i=a-'a'+2; i<=26; i++) nr+=l[i];
    return nr;
}


int main ()
{
    int n; char a;
    long long s=0;
    f>>n;
    for (int i=1; i<=n; i++)
    {
        f>>a; l[a-'a'+1]++;
        s+=MaiMariDecat(a);
    }
    g<<s<<'\n';
    return 0;
}