#include <fstream>
using namespace std;

#define MAX 10001

int n,i,j,nr;
char s[MAX];
int sg[26];

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

void citire()
{
    f>>n;
    f>>s;
}
void solve()
{
    for (i=0;i<n;i++)
    {
        ++sg[s[i]-'a'];
        for (j=s[i]-'a' + 1;j<26;j++)
            if (sg[j]!=0)
                nr+=sg[j];
    }

}
void scriere()
{
    g<<nr;
}
int main()
{
    citire();
    solve();
    scriere();
    return 0;
}