The numbers in the following sequence increase by 2, then by 4, then by 6, etc. Continue this sequence until you reach the first number that is not a prime.

17
19
23
29
37

The next number in the sequence would be 37 + 8 = 45. However, 45 is not a prime number because it is divisible by 3. Therefore, 45 is the first number in the sequence that is not a prime number.