#include <fstream>
#include <algorithm>
#define NMAX 100001
using namespace std;
ifstream f("litere.in");
ofstream g("litere.out");
int nr , i , n , j;
char c[NMAX] ;
int v[NMAX];
int main()
{
    int x = 0;
    f >> n;
    for(i = 1 ; i <= n; ++i){
        f >> c[i];
        v[i] = c[i] - 'a' + 1;
    }
    for(i = 1 ; i <= 'z' - 'a' + 1 ; ++i){
        x = 0;
        for(j = 1 ; j <= n ; ++j){
            if(v[j] == -1){
                ++x;
            }
            if(v[j] == i){
                nr += j - 1 - x;
                ++x;
                v[j] = -1;
            }
        }
    }
    g << nr;
    return 0;
}