#include <stdio.h>
#include <stdlib.h>
int main()
{
    int i,j;
    int a,b,bucket[100000];
    for (i = 0; i < 100000; i++)
    {
        bucket[i] = 1;
    }
    
    scanf("%d %d",&a,&b);

    if(abs(a - b) % 2 == 0)printf("-1");
    else
    {
        for (i = 0; i <= 10000; i++)
        {
            for (j = 0; j <= 10000; j++)
            {
                if(b * i + a * j >= 100000)break;
                bucket[b*i + a*j] = 0;
            }
        }
        int cnt = 0;
        for (i = 0; i <= a * b * 2; i++)
        {
            //if(bucket[i])printf("%d\t",i);
            cnt += bucket[i];
        }
        printf("%d",cnt);
    }
}