freeCodeCamp/curriculum/challenges/english/10-coding-interview-prep/project-euler/problem-430-range-flips.md

id, title, challengeType, forumTopicId, dashedName

id	title	challengeType	forumTopicId	dashedName
5900f51a1000cf542c51002d	Problem 430: Range flips	5	302101	problem-430-range-flips

--description--

N disks are placed in a row, indexed 1 to N from left to right.

Each disk has a black side and white side. Initially all disks show their white side.

At each turn, two, not necessarily distinct, integers A and B between 1 and N (inclusive) are chosen uniformly at random. All disks with an index from A to B (inclusive) are flipped.

The following example shows the case N = 8. At the first turn A = 5 and B = 2, and at the second turn A = 4 and B = 6.

Let E(N, M) be the expected number of disks that show their white side after M turns. We can verify that E(3, 1) = \frac{10}{9}, E(3, 2) = \frac{5}{3}, E(10, 4) ≈ 5.157 and E(100, 10) ≈ 51.893.

Find E({10}^{10}, 4000). Give your answer rounded to 2 decimal places behind the decimal point.

--hints--

rangeFlips() should return 5000624921.38.

assert.strictEqual(rangeFlips(), 5000624921.38);

--seed--

--seed-contents--

function rangeFlips() {

  return true;
}

rangeFlips();

--solutions--

// solution required