adding unique number suffix

hi, someone helped me earlier but now i have a new problem.
i need to make unique skus from col B. so i asked for a suffix to be added to main item number of column B, like 18-1, 18-2, 18-2, etc.
you'll notice in column C, there is 18-3, 18-4. for reasons unknown, i can't wipe out column c and start over using this code provided earlier for sequential numbering of each set of numbers in column B [=B2 & -COUNTIF(B$1:B2, B2)]

so, my question is, how can i leave column C intact, but for each set of numbers, add a random number to make a unique sku, IF there's not already one there in column C. also, the prefix must be column B. So, column c1 can be 18-1, c2 can be 18-2, c5 should be 19-1, etc.


thanks so much!bummer.xlsx



6893 18 Pink 0 0 Out of Stock
6894 18 Silver 1 0 In Stock
6895 18 18-3 Aqua 0 0 Out of Stock
6896 18 18-4 Purple 0 0 Out of Stock
6897 19 Black 8 0 In Stock
6898 41 Black 0 0 Out of Stock
6899 45 Tan Stripe 0 0 Out of Stock
6900 45 45-2 Gold Stripe 0 0 Out of Stock
6901 45 45-5 White Spot 0 0 Out of Stock

this worked great. thanks!