program pascal;
var f,g:text;
    n,i,j,k:longint; nr:int64;
    v:array[1..10002] of char;
    ok:boolean; aux,c:char;

  procedure citire;
  begin
   readln(f,n);
   i:=0;
   while i<n do
     begin
       read(f,c);
       if (ord(c)>96) and (ord(c)<123) then
             begin
                i:=i+1;
                v[i]:=c;
             end;
     end;
 end;

  procedure form;
  begin
 nr:=0;
 i:=2;
 while i<=n do
 begin
 ok:=false;
 aux:=v[i];
 k:=i-1;
 while v[i]<v[k] do
         begin
                 nr:=nr+1;
                 ok:=true;
                 k:=k-1;
         end;

 if ok then
                 begin
                  for j:=i downto k+2 do v[j]:=v[j-1];
                  v[k+1]:=aux;
                 end;
 i:=i+1;
 end;

 write(g,nr);

  end;

begin
 assign(f,'litere.in'); reset(f);
 assign(g,'litere.out'); rewrite(g);

  {repeat
  ok:=true;
  for i:=1 to n-1 do
  if v[i]>v[i+1] then begin
                         aux:=v[i];
                         v[i]:=v[i+1];
                         v[i+1]:=aux;
                         ok:=false;
                         nr:=nr+1;
                     end;
  until ok;}
  citire;
  form;
 close(f);
 close(g);
end.