#include <iostream>
#include <fstream>
#define MAX 32
using namespace std;
int n,fr[MAX];
string s;

ifstream fin("litere.in");
ofstream fout("litere.out");

int main()
{
    fin >> n >> s;
    int ans = 0;
    for(int i = 0; i < s.size(); i++){
        fr[s[i]-'a']++;
        for(int j = s[i]-'a'+1; j <= 'z'-'a'; j++){
            ans += fr[j];
        }
    }
    fout << ans;
    return 0;
}