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Read the original below-strike input
Strike is 100 and terminal price 90. The put holder’s favorable difference is 100−90=10. The call holder’s difference is 90−100<0 and its payoff is zero.
Inspect both subtraction order and zero floor
Using max(90−100,0) for the put produces zero and describes the wrong contract. Using 90−100 without a floor additionally permits a negative holder payoff. These are distinct errors.
| Expression | Result | Object |
|---|---|---|
| max(100−90,0) | 10 | Put holder |
| max(90−100,0) | 0 | Call holder |
| 90−100 | −10 | Unfloored difference |
Write the regions before substituting numbers
Below strike the put pays K−S; at or above strike it pays zero. A writer’s payoff then adds a separate negative sign. Reversing the contract expression is not a substitute for applying the position sign.
Use an above-strike test too
At S=120 the same put should pay zero while the call pays 20. Testing both regions confirms the contract direction better than repeating one below-strike example with new numbers.