#include <iostream>
#include <fstream>
#include <string.h>
using namespace std;

int main()
{
    ifstream cin ("litere.in");
    ofstream cout ("litere.out");
    int n, l = 1, nr = 0, a[100001];
    char s;
    cin >> n;
    for (; n; --n){
        cin >> s;
        if (isalpha(s))
            a[l ++] = (int)s;
    }
    for (int i=1; i<=l-1; i++)
        for (int j=i+1; j<=l; j++)
            if (a[i] > a[j]){
                nr ++;
                swap(a[i], a[j]);
            }
    cout << nr;
    return 0;
}