Why can't I put code in-between if and else if statements?

A generic example in Python:

def mahFuncshiown(a, b):

    c = None

    if a == 3:
        c = 8

    a += 9

    else if a + 10 == 15:
        c = 2

    return c + a

What's the reason for most programming languages not allowing code to be put in-between if and else if statements? The else if should execute if the previous if was not True.