codeforces 755 A O(N)素数筛模板

题目：

A. PolandBall and Hypothesis

time limit per test

2 seconds

memory limit per test

256 megabytes

input

standard input

output

standard output

PolandBall is a young, clever Ball. He is interested in prime numbers. He has stated a following hypothesis: "There exists such a positive integer n that for each positive integer m number n·m + 1 is a prime number".

Unfortunately, PolandBall is not experienced yet and doesn't know that his hypothesis is incorrect. Could you prove it wrong? Write a program that finds a counterexample for any n.

Input

The only number in the input is n (1 ≤ n ≤ 1000) — number from the PolandBall's hypothesis.

Output

Output such m that n·m + 1 is not a prime number. Your answer will be considered correct if you output any suitable m such that 1 ≤ m ≤ 103. It is guaranteed the the answer exists.

Examples

input

3

output

1

input

4

output

2

Note

A prime number (or a prime) is a natural number greater than 1 that has no positive divisors other than 1 and itself.

For the first sample testcase, 3·1 + 1 = 4. We can output 1.

In the second sample testcase, 4·1 + 1 = 5. We cannot output 1 because 5 is prime. However, m = 2 is okay since 4·2 + 1 = 9, which is not a prime number.

代码：

#include<bits/stdc++.h>#include<queue>using namespace std;const int maxn=1e6+30;const __int64 inf=(__int64)1<<62;int n,m;int p[maxn>>2];//存储素数 bool a[maxn];//O(n) 素数筛 void init() { memset(a,false,sizeof(a));//初始全部为素数 int num=0;for(int i=2;i<maxn;++i) {if(!a[i]) p[num++]=i;for(int j=0;(j<num&&i*p[j]<maxn);++j) {a[i*p[j]]=1;if(i%p[j] == 0) break;}}}int main() {init();while(cin>>n) {int t=n;n+=1;for(int m=1; m<=1000; ++m,n+=t) {if(a[n]) {cout<<m<<endl;break;}}}return 0;}