var v:array[1..10000] of char;
a:array[97..122] of integer;
f:text;
nr,n,i,j,ind:longint;
c:char;
begin
nr:=0;
assign(f,'litere.in');reset(f);
readln(f,n);
for i:=1 to n do read(f,v[i]);
close(f);
for i:=1 to n do
    begin
    inc(a[ord(v[i])]);
    for j:=ord(v[i])+1 to 122 do
        nr:=nr+a[j];
    end;
assign(f,'litere.out');rewrite(f);
writeln(f,nr);
close(f);
end.