var a:array[1..10000] of char;
    c:array[1..2,1..200] of integer;
    f,g:text;
    i,n,op:integer;
begin
assign(f,'litere.in'); reset(f);
assign(g,'litere.out'); rewrite(g);
read(f,n); readln(f);
for i:=1 to n do begin                          
  read(f,a[i]);
  inc(c[1,ord(a[i])]);
end;
for i:=97 to 122 do c[2,i]:=c[2,i-1]+c[1,i];
op:=0;
for i:=1 to n do
  if i>=c[2,ord(a[i])] then inc(op);
for i:=1 to n do
  if i<=c[2,ord(a[i])] then c[2,ord(a[i])]:=c[2,ord(a[i])]-1
                       else begin
                 op:=op+i-c[2,ord(a[i])];
                 c[2,ord(a[i])]:=c[2,ord(a[i])]-1;
               end;
write(g,op);
close(g);
end.