program litere;
var f,g:text;
        n:longint;
        vf:array['a'..'z'] of longint;


procedure iofile;
begin
        assign(f,'litere.in');reset(f);
        assign(g,'litere.out');rewrite(g);
        readln(f,n);
end;



procedure solve;
var sum,i:longint;
        c,car:char;
begin
        sum:=0;
        fillchar(vf,sizeof(vf),0);
        for i:=1 to n do
                begin
                        read(f,c);
                        inc(vf[c]);
                        for car:=chr(ord(c)+1) to 'z' do
                                sum:=sum+vf[car];
                end;
        close(f);
        writeln(g,sum);
        close(g);
end;


begin
        iofile;
        solve;
end.