input:2,3,5,6,7,8,9,10,11,12,13,14,15 output: 4

/*

Question: Find the missing number in an array of consecutive integers.
Example:
input:2,3,4,5,6,7,8,9,10,11,12,13,14,15 output: null
input:2,3,5,6,7,8,9,10,11,12,13,14,15 output: 4
input:2,3,4,5,6,7,8,9,10,11,12,14,15 output: 13

*/