#include <fstream>
using namespace std;
ifstream f("litere.in");
ofstream g("litere.out");
int p,nr,i,t,n,fr[256];
char ch,j;
int main()
{
    f>>n;
    for(i=1;i<=n;i++){
       f>>ch;
       for(j=ch+1;j<='z';j++)
          if(fr[j]!=0)
             nr+=fr[j];
       fr[ch]++;
    }

    g<<nr;
    return 0;
}