function pairwise(arr, arg) {
  return arr.reduce(function(prev,curr,index){
    arr.forEach(function(elem,ind){
      if (ind>index && curr+elem===arg && prev.indexOf(index)===-1 && prev.indexOf(ind)===-1) prev.push(index,ind);
    });
    return prev;
  },[]).reduce(function(cur,pre){return cur+pre;},0);
}