#include <stdio.h>
int main(){
    int A,B;
    scanf("%d %d", &A, &B);
    for(int C = 1;C < A + B; C++){
        if(A != C && B != C){
            if((A+B)%C == 0 && (B+C) % A == 0 && (C+A)%B==0){
                printf("%d",C);
                return 0;
            }
        }
    }
    printf("-1");
}