#include<algorithm>
#include<fstream>
using namespace std;
ifstream in("litere.in");
ofstream out("litere.out");
char ch;
int n,i,j,nr,a[256];
int main()
{
    in>>n;
    for(i=1;i<=n;i++){
        in>>ch;
        for(j=ch+1;j<='z';j++)
            nr+=a[j];
        a[ch]++;}
    out<<nr;
    return 0;
}