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