function pairwise(arr, arg) {
  var cloneArr = arr.slice(0);
  
  return pair = cloneArr
  .map(function(i) {
    return arg-i;
  })
  .map(function(i,index) {
    var pairIndex = cloneArr.indexOf(i); 
    
    if ( cloneArr[index] !== null && pairIndex >= 0 && pairIndex !== index) {
      cloneArr[index] = null;
      cloneArr[pairIndex] = null;
      return index+pairIndex;
    } else {
      return 0;
    }
  })
  .reduce(function(a,b) {
    return a+b;
  },0);
}