TAXI PAS

const
   fin='taxi.inp';   fon='taxi.out';
   maxm=1000;vc=100;
var
   d:array[1..4] of longint;
   n:word;

procedure enter;
var i,x:word;
begin
    readln(n);
    for i:=1 to n do
    begin
        read(x);
        inc(d[x]);
    end;
end;

procedure solve;
var s:word;
begin
    s:=0;
    s:=d[4];
    s:=s+d[3];
    d[1]:=d[1]-d[3];
    s:=s+(d[2] div 2);
    if d[2] mod 2 = 1 then
    begin
        inc(s);
        d[1]:=d[1]-2;
    end;
    if d[1]>0 then s:=s+((d[1]-1) div 4)+1;
    write(s);
end;

begin
    assign(input,fin);reset(input);
    assign(output,fon);rewrite(output);
    enter;
    solve;
    close(input);close(output);
end.