A. Dreamoon and Stairs
time limit per test
1 second
memory limit per test
256 megabytes
input
standard input
output
standard output
dreamoon wants to climb up a stair of n steps. he can climb 1 or 2 steps at each move. dreamoon wants the number of moves to be a multiple of an integer m.
What is the minimal number of steps making him climb to the top of the stairs that satisfies his condition?
Input
The single line contains two space separated integers n, m (0?
Output
Print a single integer ? the minimal number of moves being a multiple of m. If there is no way he can climb satisfying condition print ?-?1 instead.
Sample test(s)
Input
10 2
Output
Input
3 5
Output
-1
Note
For the first sample, Dreamoon could climb in 6 moves with following sequence of steps: {2, 2, 2, 2, 1, 1}.
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For the second sample, there are only three valid sequence of steps {2, 1}, {1, 2}, {1, 1, 1} with 2, 2, and 3 steps respectively. All these numbers are not multiples of 5.
题意：给两个数n,m，
现在要求爬楼梯，一次可以爬
一个或两个阶梯，要求爬完n个阶梯，
求爬的次数为m的倍数时，最少次数是多少，
若不存在答案输出-1。
做法：很明显尽量爬两步。
代码：
#include#include
