//100 puncte
#include <fstream>
#include <string>
#include <algorithm>
#include <vector>
#include <map>
using namespace std;

#define in "litere.in"
#define out "litere.out"
#define NMAX 10001

int n, nrsol;
string s;
map<char,int> f;

int main()
{
    ifstream fin ( in );
    ofstream fout ( out );
    
    fin >> n;
    fin.get();
    fin >> s;

    /*for ( i = 0; i < s.size(); ++i )
        f[s[i]]++;*/
    char j;
    int i;
    
    f[s[0]]++;
    for ( i = 1; i < s.size(); ++i )
    {
        for ( j = s[i]+1; j <= 'z'; ++j )
        {
            nrsol += f[j];
        }
        f[s[i]]++;            
    }
    
    fout << nrsol << "\n";
    fin.close();
    fout.close();
    
    return 0;
}