function knight(start, finish) {
  const s = getCoords(start)
  const f = getCoords(finish)
  return getShortestPath(s.x, s.y, f.x, f.y, new Set(), 0)
}

function getShortestPath(startX, startY, targetX, targetY, visited, len) {
  if (startX === targetX && startY === targetY) {
    return len
  }
  visited.add(`${startX}:${startY}`)
  const paths = [...getPaths(startX, startY)]
    .filter(p => isInBounds(p) && !visited.has(`${p.x}:${p.y}`))
    .map(p => getShortestPath(p.x, p.y, targetX, targetY, visited, len + 1))
    .filter(x => !Number.isNaN(x))
  return paths.length > 0 ? Math.min(...paths) : NaN
}

function getCoords(str) {
  return {
    x: 'abcdefgh'.indexOf(str[0]),
    y: Number(str[1]) - 1
  }
}

function* getPaths(x, y) {
  yield {x: x+2, y: y+1}
  yield {x: x+2, y: y-1}
  yield {x: x-2, y: y+1}
  yield {x: x-2, y: y-1}
  yield {x: x+1, y: y+2}
  yield {x: x+1, y: y-2}
  yield {x: x-1, y: y+2}
  yield {x: x-1, y: y-2}
}

function isInBounds(p) {
  return p.x >= 0 && p.x < 8 && p.y >= 0 && p.y < 8
}