Problem:
Score : 100 points

Problem Statement

You are given positive integers X and Y.
If there exists a positive integer not greater than 10^{18} that is a multiple of X but not a multiple of Y, choose one such integer and print it.
If it does not exist, print -1.

Constraints

1 ≤ X,Y ≤ 10^9

X and Y are integers.

Input

Input is given from Standard Input in the following format:

X Y

Output

Print a positive integer not greater than 10^{18} that is a multiple of X but not a multiple of Y, or print -1 if it does not exist.

Sample Input 1

8 6

Sample Output 1

16

For example, 16 is a multiple of 8 but not a multiple of 6.

Sample Input 2

3 3

Sample Output 2

-1

A multiple of 3 is a multiple of 3.

Solution in C (GCC 5.4.1) (judge verdict: Runtime Error)

#include <stdio.h>

int main(void){
    int a, b;
    while(~scanf("%d%d", &a, &b)){
        int x = 0,c[100];
        for(int i = a;i <= 1018;i++){
            if(i % a == 0 && i % b != 0){
                c[x] = i;
                x++;
            }
        }
        if(x != 0)
            printf("%d\n", c[x/b]);
        if(x == 0)
            printf("-1\n");
    }
    return 0;
}