K-3 Leninsky Komsomol Submarine is the first submarine of USSR to reach North Pole on June 17, 1962. She is one of USSR proudest heritage in history.
Requirements for this submarine papercraft is pretty low, only 4 piece of paper needed and with only 2 main color used which is Red and Black. The difficulty however would require some massive thinking from your side, they do not come with any text guide.
They just came with pictorial guide of which goes to which and you have to figure it our yourself on how to combine and mold all those parts.
Regardless for those who is into military I think K-3 is the best crafts for adults you should make, it somewhat look like the Red October starred by Sean Connery.