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(void){
    long long int N,a[N],b[N],i,suma=0,sumb=0;
     scanf("%lld",&N);
     scanf("%lld",a);
     scanf("%lld",b);
    for(i=0;i<N;i++){
     suma+=a[i];
    }
    for(i=0;i<N;i++){
     sumb+=b[i];
    }
    if(sumb-suma<0) printf("No");
    return 0;
    }