#include<fstream>
using namespace std;
ifstream cin("litere.in");
ofstream cout("litere.out");
int n,i,nr,v[257],j;
char q;
int main()
{
    cin>>n;
    for(i=1;i<=n;i++)
    {
        cin>>q;
        v[q]++;
        for(j=q+1;j<='z';j++)
           nr=nr+v[j];
    }
    cout<<nr;
    return 0;
}