Bran and Arrow(Easy)

Bran has a quiver. He can perform two actions either he can take an arrow from the quiver or add a new arrow to the quiver. He performs these action $n$ number of times. It is obvious that he cannot take out an arrow from an empty quiver. He wants to test Arya’s intelligence so he gives the sequence of action that he has performed in the form of a string $s$ and asks her the minimal possible numb