Arrange Bolcks

Majnu bhaiya has $2n$ Blocks, all blocks are placed on line at points $b_1,b_2,b_3,\dots,b_{2n}$ such that $b_i=b_{i-1}+1$ for all valid $i$. Now to save some space in his home he decided to move $n$ blocks from there original position and put it on other $n$ blocks such that there is exactly $2$ block on $n$ points. suppose $a_1,a_2,a_3,\dots,a_n$ is $n$ points where we have put $2$ blocks on e