#include<stdio.h>
int main(void){
    unsigned long long int A,B,ans,ans_ap,ans_bp,max_ans,n;
    scanf("%llu %llu%*c",&A,&B);
    if(A<B){
        ans_ap=B-A;//+=B*x
        ans_bp=A*(B/A+1)-B;//+=A*x
    }else{
        ans_ap=B*(A/B+1)-A;//+=B*x
        ans_bp=A-B;//+=A*x
    }
    max_ans=A+B;
    ans=ans_bp<ans_ap?ans_bp:ans_ap;
    while(ans<=max_ans){
        if(max_ans%ans==0&&(A+ans)%B==0&&(B+ans)%A==0&&ans!=A&&ans!=B)break;
        if(ans_ap<ans_bp){
            if(ans_ap+B<ans_bp){
                ans_ap+=B;
                ans=ans_ap;
            }else{
                ans=ans_bp;
                ans_bp+=A;
            }
        }else{
            if(ans_bp+A<ans_ap){
                ans_bp+=A;
                ans=ans_bp;
            }else{
                ans=ans_ap;
                ans_ap+=B;
            }
        }
    }
    if(ans>max_ans)printf("-1");
    else printf("%llu",ans);
}