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>

#define MAX 1000000000
#define NMAX 10000

int main(void){
    int N,a[NMAX],b[NMAX],sum_a,sum_b,i,j,red_ab;
    scanf("%d", &N);
    for(i = 0; i < N; i++){
        scanf("%d", &a[i]);
    }
    for(i = 0; i < N; i++){
        scanf("%d", &b[i]);
    }
    sum_a = 0;
    sum_b = 0;    
    for(i = 0; i < N; i++){
        sum_a += a[i];
        sum_b += b[i];
    }
    red_ab = sum_b - sum_a;
    if(red_ab < 0){
        printf("No");
        return 0;
    }
    else{
        j = 0;
        for(i = 0; i < N; i++){
            if(a[i] > b[i]){
                j += a[i] - b[i];
            }
            else if(b[i] > a[i]){
                j += (b[i] - a[i]) / 2 + (b[i] - a[i]) % 2;                
            }
            if(j >= red_ab){
                printf("No");
                return 0;
            }
        }
    }
    printf("Yes");
    return 0;
}