Numbers

Savages living on the island can only count up to two, so they use only two digits: 1 and 2. Additionally, these natives are very superstitious. They consider numbers that contain three identical consecutive digits to be unlucky, and do not use such numbers. For example, the number 1221212 is allowed because it does not contain three consecutive identical digits. However, the number 12121112 is not allowed, because it has three consecutive identical digits (in this example, they are ones).