#include<fstream>
using namespace std;
int n,i,fr[256],nr;
char ch;
ifstream f("litere.in");
ofstream g("litere.out");
int main(){
f>>n;
for(i=1;i<=n;i++){
    f>>ch;
    fr[ch]++;
    for(char j=ch+1;j<='z';j++){
        nr=nr+fr[j];

    }

}
g<<nr;


return 0;
}