Can even numbers become prime?
Jelly, &nbsp;8&nbsp; 7 bytes

ḊżṖVÆPS


A monadic link taking a sequence member and returning its index in the sequence.

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How?

ḊżṖVÆPS - Link: number, n
Ḋ       - dequeue (implicit range) = [ 2   , 3   , 4   ,... ,              n         ]
  Ṗ     - pop (implicit range)     = [   1 ,   2 ,   3 ,... ,                  n-1   ]
 ż      - zip                      = [[2,1],[3,2],[4,3],... ,             [n , n-1]  ]
   V    - evaluate as Jelly code   = [ 21  , 32  , 43  ,... ,         int("n"+"n-1") ]
    ÆP  - is prime? (vectorises)   = [  0  ,  0  ,  1  ,... , isPrime(int("n"+"n-1"))]
      S - sum