#include <fstream>
using namespace std;
ifstream f("litere.in"); ofstream g("litere.out");
int n,ap[27],sol;
char a;
int main()
{   f>>n;
    for(int i=0;i<n;++i)
    {   f>>a; ++ap[a-'a'];
        for(int j=a-'a'+1;j<='z'-'a';++j) sol+=ap[j];
    }
    g<<sol; g.close(); return 0;
}