#include <fstream>
#include <iostream>
#include <algorithm>
#include <string>
#include <math.h>

using namespace std;
#define ll long long
#define llu long long unsigned
#define pb push_bak
#define mp make_pair

string problemName = "litere";
string inFile = problemName+".in";
string outFile = problemName+".out";
ifstream fin(inFile.c_str());
ofstream fout(outFile.c_str());

int ap[26];
string s;

int main(){
    int j,n,i,ans;
    ans = 0;
    fin>>n;
    fin>>s;
    for(i = 0;i < n;i++){
        ap[s[i]-'a']++;
        for(j = s[i]-'a'+1;j < 26;j++){
            ans += ap[j];
        }
    }
    fout<<ans;
    return 0;
}