problem0123.py

def sieve_for_primes_to(n):
    size = n//2
    sieve = [1]*size
    limit = int(n**0.5)
    for i in range(1,limit):
        if sieve[i]:
            val = 2*i+1
            tmp = ((size-1) - i)//val 
            sieve[i+val::val] = [0]*tmp
    return [2] + [i*2+1 for i, v in enumerate(sieve) if v and i>0]

def find_r(n, s):
    return 2 * s[n-1] * (n)

sieve = sieve_for_primes_to(1000000)

i = 1
while (find_r(i, sieve) < 10000000000):
    i+=1

print i+1