#include <fstream>
#include <algorithm>
using namespace std;
ifstream cin("litere.in");
ofstream cout("litere.out");
const int nmax=10000;
int n;
char s[nmax+5];
int ap[30],ans;
int main()
{
    cin>>n;cin.get();
    cin.getline(s,nmax+5);
    for(int i=0;i<n;i++)
    {
        for(int j=s[i]-'a'+1;j<26;j++)
            ans+=ap[j];
        ap[s[i]-'a']++;
    }
    cout<<ans;
    return 0;
}