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: Wrong Answer)

     #include<stdio.h>
        int main(void){
        long long int X,Y,i=1,ans=-2;
        int nans=-1;
        scanf("%lld",&X);
        scanf("%lld",&Y);
        if(X<Y) ans =X;
        if(Y==1) {
        printf("%d",nans);
        return 0;
       }
        if(X>Y) {
        if(X%Y!=0) ans = X;
        if(X%Y==0&&Y==2) ans = X+2;
        if(X%Y==0&&Y!=2) ans = X+1;
      }
         if(ans!=-2) printf("%lld\n",ans);
         else printf("%d",nans);
        return 0;
        }