Answer:
It is 0. I don't know if that was a joke or not.
If 12 friends came and 13 are trapped there will be 0 as all the 12 friends got trapped and maybe with a miracle, one more friend came and also got trapped.
Explanation:
This is confusing though:
12-13= -1 people and that means there are no people left or its impossible to have -1 person unless there is more friends.
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