List the first 2N positive integers (here let N = 4):
1, 2, 3, 4, 5, 6, 7, 8
Divide them arbitrarily into two groups of N numbers:
1, 4, 6, 7
2, 3, 5, 8
Arrange one group in ascending order, the other in descending order:
1, 4, 6, 7
8, 5, 3, 2
Now the sum of the absolute differences of these pairs will always equal N2:
| 1 – 8 | + | 4 – 5 | + | 6 – 3 | + | 7 – 2 | = 16 = N2
(Presented by Vyacheslav Proizvolov as a problem in the 1985 All-Union Soviet Student Olympiads.)