Problem 321

Swapping Counters

A horizontal row comprising of 2`n` + 1 squares has `n` red counters placed at one end and `n` blue counters at the other end, being separated by a single empty square in the centre. For example, when `n` = 3.

A counter can move from one square to the next (slide) or can jump over another counter (hop) as long as the square next to that counter is unoccupied.

Let M(`n`) represent the minimum number of moves/actions to
completely reverse the positions of the coloured counters; that is, move
all the red counters to the right and all the blue counters to the
left.

It can be verified M(3) = 15, which also happens to be a triangle number.

If we create a sequence based on the values of `n` for which M(`n`) is a triangle number then the first five terms would be:

1, 3, 10, 22, and 63, and their sum would be 99.

Find the sum of the first forty terms of this sequence.

**
These problems are part of
Project Euler
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CC BY-NC-SA 2.0 UK
**