Find the Second Occurrence of "zip" in a String

Write a function that returns the position of the second occurrence of "zip" in a string, or -1 if it does not occur at least twice. Your code should be general enough to pass every possible case where "zip" can occur in a string.

Examples

findZip("all zip files are zipped") ➞ 18

findZip("all zip files are compressed") ➞ -1

Notes

Uppercase "Zip" is not the same as lowercase "zip".