program alex;
var f:text;
    n,d,k,i:integer;
    e:boolean;
    z:char;
    a:array[1..10000]of char;
begin
assign(f,'litere.in');
reset(f);
readln(f,n);
for i:=1 to n do
    read(f,a[i]);
close(f);
e:=false;
k:=1;
d:=0;
while e=false do
      begin
      e:=true;
      for i:=1 to n-k do
          if a[i]>a[i+1] then begin
                              z:=a[i];
                              a[i]:=a[i+1];
                              a[i+1]:=z;
                              e:=false;
                              d:=d+1;
                              end;
     k:=k+1;
     end;
assign(f,'litere.out');rewrite(f);
write(f,d);
close(f);
end.