
127  ZOJ Monthly, June 2013  J
There is a straight highway with N storages alongside it labeled by 1,2,3,...,N. Bob asks you to paint all storages with two colors: red and blue. Each storage will be painted with exactly one color. Bob has a requirement: there are at least M continuous storages (e.g. "2,3,4" are 3 continuous storages) to be painted with red. How many ways can you paint all storages under Bob's requirement? InputThere are multiple test cases. Each test case consists a single line with two integers: N and M (0<N, M<=100,000). Process to the end of input. OutputOne line for each case. Output the number of ways module 1000000007. Sample Input4 3 Sample Output3 Author: ZHAO, Kui 