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 (Clang 3.8.0) (judge verdict: Time Limit Exceeded)

#include <stdio.h>

int main(void){
    int N;

    scanf("%d", &N);
    printf("0\n");

    int s[100];

    scanf("%s", s);

    char fc, tc;
    int fv = 0, tv = N;

    if(s[0] == 'V'){
        return 0;
    }else if(s[0] == 'M'){
        fc = tc = 'M';
    }else{
        fc = tc = 'F';
    }
    while(1){
        int mv = (fv + tv) / 2;

        printf("%d\n", mv);
        scanf("%s", s);

        if(s[0] == 'V'){
            return 0;
        }else{
            char mc = s[0];

            if(fc == mc){
                if((mv - fv) % 2 == 0){
                    tc = mc;
                    tv = mv;
                }else{
                    fc = mc;
                    fv = mv;
                }
            }else{
                if((mv - fv) % 2 == 0){
                    fc = mc;
                    fv = mv;
                }else{
                    tc = mc;
                    tv = mv;
                }
            }
        }
    }
}