In the figure on the left, numbers have been placed in the circles. For every pair of neighbouring numbers, the sum of the pair equals the sum of the opposite numbers.
The problem is to place the digits 1 through 6 into the circles using each number as few times as possible. In the picture on the left, we used the number 3 twice.
In each of the figures below the 1 and 5 are already in place. In each case, finish the puzzle by putting the numbers 2, 3, 4 and 6 in the proper places.
(This is a simpler version of Order the numbers from The Moscow Puzzles by Boris Kordemsky)