#1 imoimc@2016-07-26 00:10:34
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program fbi;
var p,i,j,n,m:longint;
z,w,s,ss,e,q:ansistring;
begin
readln(n);
if n=0 then begin writeln('B');close(input);close(output);halt;end;
readln(z);
m:=length(z);
for i:=1 to m do
case z[i] of
'1':s:=s+'I';
'0':s:=s+'B';
end;
w:=s;
m:=m div 2;
p:=2;
repeat
e:='';ss:='';
for i:=1 to m do
begin
q:=copy(w,i*2-1,2);
if q='BB' then e:=e+'B'
else if q='II' then e:=e+'I'
else e:=e+'F';
end;
for i:=1 to m do
ss:=ss+copy(s,(i-1)*p+1,p)+copy(e,i,1);
p:=(p+1)*2;
w:=e;
s:=ss;
m:=m div 2;
until m=0;
writeln(s);
end.