Math in a Liquid logic statement

gistfile1.txt

{% if product.metafields.preorder.qty && variant.inventory_quantity <= 0 && product.metafields.preorder.qty | plus: variant.inventory_quantity >= 0 %}
    Preorders left: {{ product.metafields.preorder.qty | plus: variant.inventory_quantity }}
    <input id="add-to-cart" type="submit" class="green-btn" value="Preorder">
{% elsif %}
		<input id="add-to-cart" type="submit" class="green-btn" value="Add to Cart">
{% endif %}