# [Charcoal], <s>16</s> 13 bytes

 ＩＬ⊟⪪⭆Ａ›×ι⊕κ¹1

[Try it online!][TIO-knmevezy] Link is to verbose version of code. Requires input sorted in descending order. Edit: Saved 3 bytes after following @DominicvanEssen's advice. Explanation:

 Ａ Input array
 ⭆ Map over elements and join
 κ Current index
 ⊕ Incremented
 × Multiplied by
 ι Current value
 › Is greater than
 ¹ Literal `1`
 ⪪ Split on
 1 Literal string `1`
 ⊟ Pop last element
 Ｌ Length
 Ｉ Cast to string
 Implicitly print

[Charcoal]: https://github.com/somebody1234/Charcoal
[TIO-knmevezy]: https://tio.run/##LYo9C8IwFAB3f0XGF4ilVRCho0MXLRncSodQHiaQD01e9OfHRFyO47hNq7gFZUv5aGORwRwIpoiKMMLdOExwM9647ODFBbuif5CuWn3gnDOZk4YsmAzPVsedjMYTXFQi@M@5fmMpy9J3Z8H67tRwbDj8sK5l/7Zf "Charcoal – Try It Online"