CodeForces 755A

A - 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.

Example
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 numbe
题意找一个数和已知数相乘然后+1为非质数。
思考：可以打表然后找；
还可以用奇数乘一加1肯定为非质数。偶数乘自身减2然后+1得到（n-1)*(n-1)=非质数
代码：

#include<stdio.h>
int main()
{
    int n;
    while(scanf("%d",&n)!=EOF)
    {
    if(n==1)
    {
    printf("3\n");
    continue;
    }
    if(n==2)
    {
    printf("4\n");
        continue;}
    if(n%2==1)
    {
        printf("1\n");
    }
    else
    printf("%d\n",n-2);
}
}