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(){
  int N;
  int i;
  long long int s=0;
  scanf("%d\n",&N);
 
  int a[N],b[N];
 
  for(i=0;i<N;i++) scanf("%d ",&a[i]);
  for(i=0;i<N;i++) scanf("%d ",&b[i]);
 
  for(i=0;i<N;i++)
    if (a[i]<=b[i])
      s += (b[i]-a[i])/2;
    else
      s -= a[i]-b[i];
  if(s>=0)
    printf("Yes\n");
  else
    printf("No\n");
 
  return 0;
}