function pairwise(arr, arg) {
  var index = [];
  for (var i=0;i<=arr.length-1;i++)
    for (var j=i+1;j<=arr.length-1;j++)
      if ( !index[j] && !index[i])
        if (arr[j]+arr[i]==arg)
          index[j] = index[i]= true;
  //console.log(index);
  return index.reduce(function(sum, item, index){
    if(item) return sum+index;
  },0)

}

pairwise([1,4,2,3,0,5], 7);