#include <stdio.h>
#include <string>
#include <cstring>
#include <stdlib.h>
#include <math.h>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <list>
#include <iterator>
#pragma warning(disable:4996)
 
typedef long long ll;
#define MIN(a, b) ((a)>(b)? (b): (a))
#define MAX(a, b) ((a)<(b)? (b): (a))
#define LINF 9223300000000000000
#define INF 2140000000
#define MOD 1000000007
using namespace std;

ll gcd(ll a, ll b) {
    if(b == 0) return a;
    return gcd(b,a%b);
}
 
int main(int argc, char* argv[])
{
    ll a,b;
    scanf("%lld%lld", &a, &b);
    if(a==b) {
        printf("%ld\n", a);
    }
    else {
        ll a0=MIN(a,b), b0=MAX(a,b);
        ll d=gcd(a0,b0);
        ll a1=a0/d, b1=b0/d;
        long k;
        for(k=1; k<=2; k++) {
            ll c1=k*a1*b1-a1-b1;
            if(c1>0 && (a1+b1+c1)%a1==0 && (a1+b1+c1)%b1==0 && (a1+b1+c1)%c1==0) {
                printf("%lld\n", c1*d); return 0;
            }
        }
        printf("-1\n");
    }

    return 0;
}