function getMinimumCost(k, c) {
  let min = 0;

  // sort flowers - highest to lowest
  c.sort((a, b) => (a - b < 0 ? 1 : -1));

  for (let buyerIdx = 0; buyerIdx < k; buyerIdx++) {  // e.g. 3 friends = 3 buyers
    // with every purchase the next purchased flower gets multiplied
    let multiplier = 1;

    // to have the lowest possible multiplier on the hightest prices,
    //  -> buy every "k" flower (flowerIdx += k) in the price-sorted array
    for (let flowerIdx = buyerIdx; flowerIdx < c.length; flowerIdx += k) {
      min += c[flowerIdx] * multiplier++;
    }
  }

  return min;
}