In men's public toilets with urinals, there is this unwritten rule that you leave at least one urinal free between you and the next person peeing. For example if there are 3 urinals and one person is already peeing in the left one, you will choose the urinal on the right and not the one in the middle. That means that a maximum of 3 people can pee at the same time on public toilets with 5 urinals when following this rule (Only 2 if the first person pees into urinal 2 or 4).
You need to write a function that returns the maximum of free urinals as an integer according to the unwritten rule.
A String containing 1s and 0s (Example: 10001) (1 <= Length <= 20)
A one stands for a taken urinal and a zero for a free one.
10001 returns 1 (10101)
1001 returns 0 (1001)
00000 returns 3 (10101)
0000 returns 2 (1001)
01000 returns 1 (01010 or 01001)
When there is already a mistake in the input string (for example 011), then return -1