Start counting at 17 for EIP-663

EIPS/eip-663.md

@@ -36,7 +36,7 @@ If the code is legacy bytecode, both of these instructions result in an *excepti
 
 If the code is valid EOF1, the following execution rules apply:
 
- 1. These instructions are followed by an 8-bit immediate value, which we call `imm`, and can have a value of 0 to 255. We introduce the variable `n` which equals to `imm + 1`.
+ 1. These instructions are followed by an 8-bit immediate value, which we call `imm`, and can have a value of 0 to 255. We introduce the variable `n` which equals to `imm + 17`.
 
  2. For `DUPN`:
    - If the current stack depth is less than `n`, then a stack underflow exception is issued.
@@ -53,7 +53,7 @@ The gas cost for both instructions is set at 3.
 
 ## Rationale
 
-TBA
+The offset 17 was chosen to avoid these new instructions overlapping with the existing `DUP?` and `SWAP?` instructions, which are cheaper to deploy because they take a single byte. Having the overlap provides little benefit, and likely would see very little use.
 
 ## Backwards Compatibility
 
@@ -65,7 +65,7 @@ TBA
 
 ## Security Considerations
 
-The authors are not aware of any additional risks introduced here. The EVM stack is fixed at 1024 items and most implementations keep that in memory at all times. This change will increase the easy-to-access number of items from 16 to 256.
+The authors are not aware of any additional risks introduced here. The EVM stack is fixed at 1024 items and most implementations keep that in memory at all times. This change will increase the easy-to-access number of items from 16 to 274.
 
 ## Copyright