#include <fstream>

#define DMAX 26
#define MAX 10005

using namespace std;

char sir[MAX];
int a[DMAX], n;

int main()
{
    ifstream in("litere.in"); in>>n; in.get(); in.getline(sir, MAX); in.close();
    long long result = 0;
    for(int i = 0; i < n; i++)
    {
        result += a[sir[i] - 'a'];
        for(int j = sir[i] - 'a' - 1; j >= 0; j--) a[j]++;
    }
    ofstream out("litere.out"); out<<result; out.close();
    return 0;
}