#include <iostream>
#include <fstream>
#include <vector>
#include <algorithm>
#include <cstring>
#include <unordered_map>
//#define CONSOLE /// daca ai in consola
#define len strlen(a)
#define int long long
using namespace std;
 
ifstream fin("litere.in");
ofstream fout("litere.out");
 
#ifdef CONSOLE
    #define fin cin 
    #define fout cout 
#endif
int f[26];
signed main() { 
    int n,nr=0;
    string s;
    fin>>n;
    fin>>s;
    for(int i=0;i<n;i++){
        for(char j=s[i]+1;j<='z';j++)nr+=f[j-'a'];
        f[s[i]-'a']++;
    }
    fout<<nr;
    return 0;
}