var n,nr:longint;
    i,aux:byte;
    c:char;
    f:text;
    ok:boolean;
    a:array[1..10000] of byte;
begin
assign(f,'litere.in');reset(f);
readln(f,n);
for i:=1 to n do begin
                 read(f,c);
                 a[i]:=ord(c);
                 end;
close(f);
nr:=0;
repeat
  ok:=true;
  for i:=1 to n-1 do
      if a[i]>a[i+1] then begin
                          ok:=false;
                          inc(nr);
                          aux:=a[i];
                          a[i]:=a[i+1];
                          a[i+1]:=aux;
                          end;
until ok;
assign(f,'litere.out');rewrite(f);
write(f,nr);
close(f);
end.