The prime number sequence starts with: `2,3,5,7,11,13,17,19...`. Notice that `2` is in position `one`. 
`3` occupies position `two`, which is a prime-numbered position. Similarly, `5`, `11` and `17` also occupy prime-numbered positions. We shall call primes such as `3,5,11,17` dominant primes because they occupy prime-numbered positions in the prime number sequence. Let's call this `listA`. 
As you can see from listA, for the prime range `range(0,10)`, there are `only two` dominant primes (`3` and `5`) and the sum of these primes is: `3 + 5 = 8`. 
Similarly, as shown in listA, in the `range (6,20)`, the dominant primes in this range are `11` and `17`, with a sum of `28`.
Given a `range (a,b)`, what is the sum of dominant primes within that range? Note that  `a <= range <= b` and `b` will not exceed `500000`.
Good luck!
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def solve(a,b):

    
    range_n = b+1 
    is_prime = [True] * range_n
    primes = [0]
    result = 0
    for i in range (2, range_n):
        if is_prime[i]:
            primes += i,
            for j in range (2*i, range_n, i):
                is_prime[j] = False

    for i in range (2, len(primes)):
        if primes[i] >= a and is_prime[i]: result += primes[i]
            
    return result
def solve(a,b):

    
    range_n = b+1 
    is_prime = [True] * range_n
    primes = [0]
    result = 0
    for i in range (2, range_n):
        if is_prime[i]:
            primes += i,
            for j in range (2*i, range_n, i):
                is_prime[j] = False

    for i in range (2, len(primes)):
        if primes[i] >= a and is_prime[i]: result += primes[i]
            
    return result