21. There is 1 long enough rectangular strip of paper, one side of the strip is black, and the other side is red. If 1 strip of paper is folded to form a red and black alternating "cross" pattern, then the minimum number of folds required to form the "cross" pattern is $\qquad$ times (folding is allowed to overlap, and excess parts and small gaps do not need to be considered).
answer: 3