0 to put something together with something else, making the whole thing bigger -- ~を加える, 追加する
1 to put two or more numbers together to get a total -- (数)を足す
2 to say another thing -- (言葉)を付け加える, ~を言い足す
We can complete it step by step however long the road is and it can't be completed however short the road is if you don't even mark your footprint.
道路がどれだけ長くても、一歩一歩完了することができますし、足跡をマークしなければ、道路がどんなに短くても完了することはできません。