--description--

Starting in the top left corner of a 2×2 grid, and only being able to move to the right and down, there are exactly 6 routes to the bottom right corner.

How many such routes are there through a given gridSize?

--hints--

latticePaths(4) should return a number.

assert.isNumber(latticePaths(4));

latticePaths(4) should return 70.

assert.strictEqual(latticePaths(4), 70);

latticePaths(9) should return 48620.

assert.strictEqual(latticePaths(9), 48620);

latticePaths(20) should return 137846528820.

assert.strictEqual(latticePaths(20), 137846528820);

--seed--

--seed-contents--

function latticePaths(gridSize) {

  return true;
}

latticePaths(4);

--solutions--

function latticePaths(gridSize) {
  let paths = 1;

  for (let i = 0; i < gridSize; i++) {
    paths *= (2 * gridSize) - i;
    paths /= i + 1;
  }
  return paths;
}