#include <fstream>

using namespace std;

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

int n,i,j;
char c[10001];
int rez;

int main()
{
    f>>n>>c;

    for (i=0;i<n;i++)
        for (j=0;j<i;j++)
            if (int(c[j]) > int(c[i]))
                rez++;

    g<<rez;

    return 0;
}