Calculate the factorial of a non-negative integer. The factorial of a number is the product of all positive integers from 1 to that number. Return -1 for negative numbers. Note: 0! equals 1.

Examples:

Input: num = 5
Output: 120
Explanation: 5! = 5 * 4 * 3 * 2 * 1 = 120

Input: num = 0
Output: 1
Explanation: The factorial of 0 is defined as 1

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