codeforces 937B Vile Grasshoppers

http://www.elijahqi.win/2018/02/26/codeforces-937b-vile-grasshoppers/ ‎

The weather is fine today and hence it’s high time to climb the nearby pine and enjoy the landscape.

The pine’s trunk includes several branches, located one above another and numbered from 2 to y. Some of them (more precise, from 2 to p) are occupied by tiny vile grasshoppers which you’re at war with. These grasshoppers are known for their awesome jumping skills: the grasshopper at branch x can jump to branches .

Keeping this in mind, you wisely decided to choose such a branch that none of the grasshoppers could interrupt you. At the same time you wanna settle as high as possible since the view from up there is simply breathtaking.

In other words, your goal is to find the highest branch that cannot be reached by any of the grasshoppers or report that it’s impossible.

Input
The only line contains two integers p and y (2 ≤ p ≤ y ≤ 109).

Output
Output the number of the highest suitable branch. If there are none, print -1 instead.

Examples
Input

Copy
3 6
Output
5
Input

Copy
3 4
Output
-1
Note
In the first sample case grasshopper from branch 2 reaches branches 2, 4 and 6 while branch 3 is initially settled by another grasshopper. Therefore the answer is 5.

It immediately follows that there are no valid branches in second sample case.

全场这题被坑了很久 但是后来想想因为1e9之内素数有5e7 所以平均不超过100个 就可能有答案 直接从最高位开始暴力枚举即可 注意判断一下这个因数是否小于等于p

#include<cmath>
#include<cstdio>
int p,y;
int main(){
//  freopen("b.in","r",stdin);
    scanf("%d%d",&p,&y);
    int ans=0;
    while(y>p){
        bool flag=0;
        for (int i=2;i<=sqrt(y);++i){
            if ((y%i==0)&&i<=p) {flag=1;break;}
        }if (!flag) {ans=y;break;}y--;
    }if (!ans) puts("-1");else printf("%d\n",ans);
    return 0;
}