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program p1114;
var
 n:integer;
 s:ansistring;

procedure search(s:ansistring);
var
 l:integer;
 n0,n1:boolean;
begin
 l:=length(s);
 if l>1 then
 begin
  search(copy(s,1,l div 2));
  search(copy(s,l div 2+1,l div 2));
 end;
 if pos('0',s)<>0 then n0:=true else n0:=false;
 if pos('1',s)<>0 then n1:=true else n1:=false;
 if n0 and n1 then write('F')
    else if n0 then write('B')
            else write('I');
end;

begin
 readln(n);
 readln(s);
 search(s);
end.