First, triviality of 1 is 0.
Second, we can judge number i from 2 to 1000000, if i is a prime number, then prime[i] is true.
Third, we traversal i from b to a, because if j>i and prime[j]==true, prime[i]==true, we can get 1/j<1/i.
if we get prime[i]==true,just output it and break.
if we can not get such a number, calculate triviality of i and output the minimum.

Thus, we can get AC with a little time spended,but larger memory.

My English is poor, if you cannot understand me, I would say sorry for it.^__^

your english are good)
i got it.
may be i got it because my solution is similar yours=)

but i got AC using 0.1s

Your english good enouth to speak with Russians)