#include <iostream>
#include <fstream>
#include <string.h>
using namespace std;
int f[127];
int main()
{
    ifstream cin ("litere.in");
    ofstream cout ("litere.out");
    int n, r = 0;
    char s;
    cin >> n;
    while (n--){
        cin >> s;
        for (int i=s+1; i<='z'; i++)
            r += f[i];
        f[s] ++;
    }
    cout << r;
    return 0;
}