Umbrella Challenge: https://gist.github.com/lubien/1f09a53a4b5607377166c58a7eb49ae0

umbrella.js

const solve = (people, umbrellas, result = 0) => {
  if (!people) return result;

  const bestUmbrellaToPick = umbrellas.reduce((best, umbrella) => {
    return umbrella > best && umbrella <= people ? umbrella : best
  }, 0);

  if (!bestUmbrellaToPick) return -1;

  const quantity = parseInt(people/bestUmbrellaToPick);

  return solve(people - bestUmbrellaToPick*quantity, umbrellas, result + quantity);
}

const tests = [{
  n: 3,
  umbrellas: [1, 2],
  expect: 2
}, {
  n: 10,
  umbrellas: [3, 1],
  expect: 4
}, {
  n: 3,
  umbrellas: [2],
  expect: -1
}];

tests.forEach(({ n, umbrellas, expect }) =>
  console.log(`solve(${n}, [${umbrellas}])`)        ||
  console.log(`   Expects: ${expect}`)              ||
  console.log(`   Output:  ${solve(n, umbrellas)}`) ||
  console.log()
);