max（与公约数相关的变形题）

题目：

Description
Give two positive integer c, n. You need to find a pair of integer (a, b) satisfy 1 ≤ a, b ≤ n and the greatest common division of a and b is c.

And you need to maximize the product of a and b

Input
The first line has two positive integer c, n

1 ≤ c, n ≤ 109

Output
Output the maximum product of a and b.

If there are no such a and b, just output -1

Sample Input
2 4

Sample Output
8

Hint
a=2, b=4

Source
牛客网2018暑期多校训练营#5

大致题意：

在1~n范围内找出a、b，且c为a和b公约数，求a和b乘积的最大值，若不存在则输出-1。

题解：

由n/c算出有几个符合条件的数，若n/c为1则将最后（即最大那个）的数平方，若n/c大于等于2则取最后两个数相乘，为0则不存在。

Code

#include
using namespace std;
int main()
{
    long long c,n,a,b,ans;
    while(~scanf("%lld%lld",&c,&n))
    {
        long long p=n/c;
        if(p==1)ans=p*c*p*c;
        else if(p>=2)ans=p*c*(p-1)*c;
        else ans=-1;
        printf("%lld\n",ans);
    }
    return 0;
}