How can I subtract one from the display of cart count?

Hello everyone,

We are looking for a solution to withdraw one item from the cart count. (When 2 items, only display “Cart (1)”

We currently have an app installed (One Tree Planted), that automatically adds an item (free tree to be planted) in cart.
This means that when a client arrives on our website, his default cart already has 1 item in it (tree).

At the moment we managed to hide this item in the cart, and only display it at checkout. Now we just need to subtract -1 to each cart count, so when a client arrives on our website, cart is empty by default.
We believe this should be fixed with some Javascript.

Thanks a lot for your precious help!

Our store url is : http://atelier-doravante.myshopify.com
Password : Dessein19129410!

You can change the cart liquid file, just filter the free item by its tag or vendor.