/**
 * Whenever we are trying to deal with a *datastructure* manipulation
 * we should never *skip, mutate, hide* the intermediary *stages* of the manipulation
 * we should try to *transform* structure from one form to another explicity
 *
 * This way we can be confident on *how*(Declarative) the datastructure has transformed
 *
 * Also, allows us to *re-use* functionality for a slightly varied datastructure
 * Eg: I can reuse the functionality for [{},{}] where sequence would be determined
 * by `id` prop of the object. In this case i would only need to change `isInSeq` implementation
 *
 * Trasnformations:
 *
 * "1,2,3,4,10,11,12,3,6,7"
 * [[1,2,3,4],[10,11,12],[3],[6.7]]
 * ["1-4", "10-12", "3", "6-7"]
 * "1-4,10-12, 3, 6-7"
 *
 * Don't worry about looping thru the array everytime. JS engine is fast
 * if u are worried, use *Structural sharing* libs like `mori, immutable.js`
 * But don't sacrifice a comprehensible code
 *
 */

var isInSeq = (current, next) => (current + 1 == next);
var getLastItem = (arr) => arr[arr.length -1];

var sequenceReducer = (acc=[], item) => {
  let chunk = getLastItem(acc);
  if(chunk && isInSeq(getLastItem(chunk), item)) {
    chunk.push(item)
  }else{
    acc.push([item])
  }

  return acc;

}

var compress = (arr=[]) => (arr.length != 1)
  ? [arr[0], getLastItem(arr)].join('-')
  : arr.join();



function main(input) {
  return input.split(',')
    .map(Number)
    .reduce(sequenceReducer, [])
    .map(compress)
    .join(",")

}