Score : 1300 points
We have a pyramid with N steps, built with blocks. The steps are numbered 1 through N from top to bottom. For each 1≤i≤N, step i consists of 2i-1 blocks aligned horizontally. The pyramid is built so that the blocks at the centers of the steps are aligned vertically.
A pyramid with N=4 steps
Snuke wrote a permutation of (1, 2, ..., 2N-1) into the blocks of step N. Then, he wrote integers into all remaining blocks, under the following rule:
Writing integers into the blocks
Afterwards, he erased all integers written into the blocks. Now, he only remembers that the permutation written into the blocks of step N was (a_1, a_2, ..., a_{2N-1}).
Find the integer written into the block of step 1.
The input is given from Standard Input in the following format:
N a_1 a_2 ... a_{2N-1}
Print the integer written into the block of step 1.
4 1 6 3 7 4 5 2
4
This case corresponds to the figure in the problem statement.
2 1 2 3
2