为什么我的程序只有80分啊？

type

rec=record

left,right:longint;

data:char;

end;

var

a:array[1..1100]of rec;

n:longint;

s:string;

t:longint;

procedure create(k:string);

var

if0,if1:boolean;

i:longint;

left,right:string;

num:longint;

begin

num:=t;

if length(k)=1 then

begin

a[t].left:=0;

a[t].right:=0;

if k='1' then a[t].data:='I' else a[t].data:='B';

exit;

end;

if0:=false;

if1:=false;

for i:=1 to length(k) do

begin

if k[i]='0'then if0:=true;

if k[i]='1'then if1:=true;

end;

if (if0)and(if1)then a[t].data:='F';

if (if1)and(not if0) then a[t].data:='I';

if (if0)and(not if1) then a[t].data:='B';

left:=copy(k,1,length(k) div 2);

right:=copy(k,length(k) div 2+1,length(k)-(length(k) div 2));

t:=num*2;

a[num].left:=t;

create(left);

t:=num*2+1;

a[num].right:=t;

create(right);

end;

procedure work(i:longint);

begin

if (a[i].left=0)and(a[i].right=0)then

begin write(a[i].data);exit;end;

if a[i].left0 then work(a[i].left);

if a[i].right0 then work(a[i].right);

write(a[i].data);

end;

begin

readln(n);

readln(s);

t:=1;

create(s);

work(1);

writeln;

end.