import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    long a = sc.nextLong();
    long b = sc.nextLong();
    long ans = (long)Math.pow(10, 18) + 1;
    if(a > b) {
      // 2x, x
      if(a == (2 * b)) ans = Math.min(ans, 3 * b);
      // 3x, x
      if(a == (3 * b)) ans = Math.min(ans, 2 * b);
      // 3x, 2x
      if((a % 3 == 0) && (b % 2 == 0) && ((a / 3) == (b / 2))) ans = Math.min(ans, b / 2);
    } else {
      // x, 2x
      if(b == (2 * a)) ans = Math.min(ans, 3 * a);
      // x, 3x
      if(b == (3 * a)) ans = Math.min(ans, 2 * a);
      // 2x, 3x 
      if((b % 3 == 0) && (a % 2 == 0) && ((b / 3) == (a / 2))) ans = Math.min(ans, a / 2);     
    }
    if(ans == ((long)Math.pow(10, 18) + 1)) ans = -1;
    System.out.println(ans);
  }
}