#include <iostream>
#include <cassert>
#include <climits>
#include <bitset>
#include <stack>
#include <queue>
#include <iomanip>
#include <limits>
#include <string>
#include <cmath>
#include <set>
#include <map>
#include <math.h>
#include <algorithm>
#include <vector>
#include <string.h>
#include <tuple>
#include <time.h>
#include <random>

using namespace std;

typedef long long ll;
typedef pair<ll,ll> P;

long long int INF = 3e18;
double Pi = 3.1415926535897932384626;

vector<ll> G[500005];
vector<P> tree[500010];
priority_queue <ll> pql;
priority_queue <P> pqp;
//big priority queue
priority_queue <ll,vector<ll>,greater<ll> > pqls;
priority_queue <P,vector<P>,greater<P> > pqps;
//small priority queue
//top pop

int dx[8]={1,0,-1,0,1,1,-1,-1};
int dy[8]={0,1,0,-1,1,-1,-1,1};
char dir[] = "DRUL";
//ll bit[500005];
//↓,→,↑,←


#define p(x) cout<<x<<endl;
#define el cout<<endl;
#define pe(x) cout<<(x)<<" ";
#define ps(x) cout<<fixed<<setprecision(25)<<x<<endl;
#define pu(x) cout<<(x);
#define pd(x) cerr<<"//"<<(x)<<endl;
#define re(i,n) for(i=0;i<n;i++);
#define pb push_back
#define lb lower_bound
#define ub upper_bound
#define deba(x) cout<< #x << " = " << x <<endl


ll mod = 1000000007;
ll rui(ll number1,ll number2){
    if(number2 == 0){
        return 1;
    }else{
        ll number3 = rui(number1,number2 / 2);
        number3 *= number3;
        number3 %= mod;
        if(number2%2==1){
            number3 *= number1;
            number3 %= mod;
        }
        return number3;
    }
}
ll gcd(ll number1,ll number2){
    if(number1 > number2){
        swap(number1,number2);
    }
    if(number1 == 0 || number1 == number2){
        return number2;
    }else{
        return gcd(number2 % number1,number1);
    }
}
void YES(bool condition){
    if(condition){
        p("YES");
    }else{
        p("NO");
    }
    return;
}
void Yes(bool condition){
    if(condition){
        p("Yes");
    }else{
        p("No");
    }
    return;
}

ll n,m,num,sum,ans,a,b,c,d,e,g,h,w,i,j,k,l,q;
ll x[500005],y[500005],z[500005];
ll dp[3000005],res[3000005];
char s[500005];
bool flag,dame;

ll  fact[400005];
ll rfact[400005];

void c3_init(){
    fact[0] = rfact[0] = 1;
    for(ll i=1; i<400000; i++){
        fact[i] = (fact[i-1]*i) % mod;
        rfact[i] = rui(fact[i],mod - 2);
    }
}
ll ncr(ll n,ll r){
    return (((fact[n] * rfact[r]) % mod) * rfact[n-r]) % mod;
}

int main(){
	cin >> a >> b;
	assert(1 <= a && a <= 1000000000);
	assert(1 <= b && b <= 1000000000);
	for(i=1;i*i<=a+b;i++){
		if((a+b)%i == 0){
			if((a+i)%b==0 && (b+i)%a==0 && a != i && b != i)x[num++] = i;
			if((a+(a+b)/i)%b == 0 && (b+(a+b)/i)%a == 0 && a != (a+b)/i && b != (a+b)/i)x[num++] = (a+b)/i;
		}
	}
	sort(x,x+num);
	if(num == 0){
		p(-1);
	}else{
		p(x[0]);
	}
	return 0;
}