Primal Sport
题意翻译
越学越掂是个贪玩的孩子。
这天,越学越掂和他的**一个**朋友正在玩游戏,轮流来的,首先给出一个$X0$,选取一个$P1$($P1$比当前数值小),然后让找$P1$的某一倍数使得它刚好大于等于当前数值(等于的时候是说这个质数本身就是当前数值的因数),这个数字便是下一个数字。再如是进行一轮。题目给出$X2$(这个数字是个非质数),希望你能求出满足要求的最小的$X0$。
题目描述
Alice and Bob begin their day with a quick game. They first choose a starting number $ X_{0}>=3 $ and try to reach one million by the process described below.
Alice goes first and then they take alternating turns. In the $ i $ -th turn, the player whose turn it is selects a prime number smaller than the current number, and announces the smallest multiple of this prime number that is not smaller than the current number.
Formally, he or she selects a prime $ p<X_{i-1} $ and then finds the minimum $ X_{i}>=X_{i-1} $ such that $ p $ divides $ X_{i} $ . Note that if the selected prime $ p $ already divides $ X_{i-1} $ , then the number does not change.
Eve has witnessed the state of the game after two turns. Given $ X_{2} $ , help her determine what is the smallest possible starting number $ X_{0} $ . Note that the players don't necessarily play optimally. You should consider all possible game evolutions.
输入输出格式
输入格式
The input contains a single integer $ X_{2} $ ( $ 4<=X_{2}<=10^{6} $ ). It is guaranteed that the integer $ X_{2} $ is composite, that is, is not prime.
输出格式
Output a single integer — the minimum possible $ X_{0} $ .
输入输出样例
输入样例 #1
14
输出样例 #1
6
输入样例 #2
20
输出样例 #2
15
输入样例 #3
8192
输出样例 #3
8191
说明
In the first test, the smallest possible starting number is $ X_{0}=6 $ . One possible course of the game is as follows:
- Alice picks prime 5 and announces $ X_{1}=10 $
- Bob picks prime 7 and announces $ X_{2}=14 $ .
In the second case, let $ X_{0}=15 $ .
- Alice picks prime 2 and announces $ X_{1}=16 $
- Bob picks prime 5 and announces $ X_{2}=20 $ .