D. [POJ 2689]Prime Distance

The branch of mathematics called number theory is about properties of numbers. One of the areas that has captured the interest of number theoreticians for thousands of years is the question of primality. A prime number is a number that is has no proper factors (it is only evenly divisible by 1 and itself). The first prime numbers are 2,3,5,7 but they quickly become less frequent. One of the interesting questions is how dense they are in various ranges. Adjacent primes are two numbers that are both primes, but there are no other prime numbers between the adjacent primes. For example, 2,3 are the only adjacent primes that are also adjacent numbers. Your program is given 2 numbers: L and U (1<=L< U<=2,147,483,647), and you are to find the two adjacent primes C1 and C2 (L<=C1< C2<=U) that are closest (i.e. C2-C1 is the minimum). If there are other pairs that are the same distance apart, use the first pair. You are also to find the two adjacent primes D1 and D2 (L<=D1< D2<=U) where D1 and D2 are as distant from each other as possible (again choosing the first pair if there is a tie).

题目大意：输入两个数L和U(1<=L<U<=2 147 483 647)，要找出两个相邻素数C1和C2(L<=C1<C2<=U)是距离最小的，如果相邻素数不止一对，选择最初的，还要找出两个相邻的素数D1，和D2是距离最大的（同样在有多对的情况下选择最初的） 其中L<U，L和U的差不超过1 000 000

Each line of input will contain two positive integers, L and U, with L < U. The difference between L and U will not exceed 1,000,000.

For each L and U, the output will either be the statement that there are no adjacent primes (because there are less than two primes between the two given numbers) or a line giving the two pairs of adjacent primes.