# combinatorics – Show that \$1= sum_{k=0}^{m} (-1)^k {m choose k}2^{m-k}\$ using sign reversing involution

Using the sign reversing involution, how can I show that $$1= sum_{k=0}^{m} (-1)^k {m choose k}2^{m-k}.$$
I have been trying to figure out the what the signed sets are namely $$S^{ +}$$ and $$S^{ -}$$. Can anyone please give me a hint to what the signed sets should be and perhaps the involution map defined on the disjoint union of the two signed sets?