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,ans=-2;
int nans=-1;
scanf("%lld",&X);
scanf("%lld",&Y);
for(i=1;i<=1000000000;i++){
 if(((i%X)==0)&&((i%Y)!=0)) {
 ans=i;
 break;
 }
 if(X<Y){
 if(Y%X!=0) {
 ans =X;
 break;
 }
 else{
 if(X+Y<=1000000000000000000){
 ans = X+Y;
 break;
 }
 else break;
 }
} 
 if(X==Y) break;
 if(X>Y) {
 ans = X;
 break;
 }
}
 if(ans!=-2) printf("%lld\n",ans);
 else printf("%d",nans);
return 0;
}