#include<fstream>
#include<vector>
#include<string>

using namespace std;

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

string sir;
int i, n, k, j,contor,st,dr,sol,x,y;
char ch;
int frec[250];
int sum[250];

int main()
{
    sol = 0;
    fin >> n;
    for(i = 1; i <= n; i++ )
    {
        fin >> ch;
        sol = sol + (sum['z'] - sum[ch]);
        frec[ch]++;
        for(j = 'a'; j <= 'z'; j++)
        {
            sum[j] = sum[j - 1] + frec[j];
        }
    }
    fout << sol << "\n";
}