var a:array[1..30000] of char;
b:array[1..10000] of integer;
aux,n,i,j,l,k,m:integer;
f,g:text;
begin
assign(f,'litere.in');reset(f);
assign(g,'litere.out');rewrite(g);
readln(f,n);
for i:=1 to n do
read(f,a[i]);
for i:=1 to n do
b[i]:=(ord(a[i]));
m:=0;
for k:=1 to n-1 do
for l:=k+1 to n do
if b[k]>b[l] then
begin
aux:=b[k];
b[k]:=b[l];
b[l]:=aux;
inc(m);
end;
write(g,m);
close(f);
close(g);
end.