I know that f(n) can be formed by adding 1 or 2 in front of f(n-1),but I dont see any order in addings.If you know,please tell me.

AFAIK, I solved it looking at base-2 representation of 122221212121...(base-10). With each step you multiply it by 10 and add 1 or 2 (next decimal digit from left to right). You want the result to become 10000000000000...000 (base-2).