Score : 600 points
Takahashi has N balls with positive integers written on them. The integer written on the i-th ball is A_i. He would like to form some number of pairs such that the sum of the integers written on each pair of balls is a power of 2. Note that a ball cannot belong to multiple pairs. Find the maximum possible number of pairs that can be formed.
Here, a positive integer is said to be a power of 2 when it can be written as 2^t using some non-negative integer t.
Input is given from Standard Input in the following format:
N A_1 A_2 ... A_N
Print the maximum possible number of pairs such that the sum of the integers written on each pair of balls is a power of 2.
3 1 2 3
1
We can form one pair whose sum of the written numbers is 4 by pairing the first and third balls. Note that we cannot pair the second ball with itself.
5 3 11 14 5 13
2