#include <fstream>
#include <algorithm>

using namespace std;

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

int n, nrInv, temp, j;
int v[31];
char c;

int main()
{
    in >> n;
    for(int i = 0; i < n; i ++){
        in >> c;
        for (j = c - 'a' + 1; j <= 'z' - 'a'; j ++){
            nrInv += v[j];
        }
        v[c - 'a'] ++;
    }
    out << nrInv;
    
    return 0;
}