文章标题 CoderForces 755A ： PolandBall and Hypothesis（水）

PolandBall and Hypothesis

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.
题意：给你一个数n,然后找出另一个数m使得n*m+1不是素数，将m输出来即可。
分析：直接从1开始遍历直到找到一个符合题意的即可
代码：

#include<iostream>
#include<string>
#include<cstdio>
#include<cstring>
#include<vector>
#include<math.h>
#include<map>
#include<queue> 
#include<algorithm>
using namespace std;
const int inf = 0x3f3f3f3f;
int n; 
int judge (int m){
    int t=m*n+1;
    for (int i=2;i*i<=t;i++){
        if (t%i==0)return 1;//不是素数，返回1 
    }
    return 0;//是素数，返回0 
}
int main ()
{
    while (cin>>n){
        int m=1;
        while (1){
            if (judge(m))break;//找到一个不是素数的m值 
            else m++;
        }
        cout<<m<<endl;
    }       
    return 0;
}