#include <iostream>
#include <fstream>
#define MAX 10002
using namespace std;
int n,f[MAX];
char v[MAX];

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

int main()
{
    fin >> n;
    for(int i = 1; i <= n; i++){
        fin >> v[i];
    }
    for(int i = 1; i <= n; i++){
        for(int j = i-1; j >= 1; j--){
            if(v[i] < v[j]){
                f[v[i]-'a'+1]++;
            }
        }
    }
    int ans = 0;
    for(int i = 1; i <= 26; i++){
        ans += f[i];
    }
    fout << ans << "\n";
    return 0;
}