import java.io.InputStream;
import java.io.PrintWriter;
import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.PriorityQueue;
import java.util.Scanner;
import java.util.Set;
import java.util.Stack;
import java.util.TreeMap;
import java.util.TreeSet;

import static java.util.Comparator.*;

public class Main {
    public static void main(String[] args) {
        PrintWriter out = new PrintWriter(System.out);
        Solver solver = new Solver(System.in, out);
        solver.solve();
        out.close();
    }
}
class Solver {
	Scanner sc;
	PrintWriter out;
    public Solver(InputStream in, PrintWriter out) {
    	sc = new Scanner(in);
    	this.out = out;
    }
    // ==================================================================
    public void solve() {
    	int A = Integer.parseInt(sc.next());
    	int B = Integer.parseInt(sc.next());
    	int wk;
    	Set<Integer> set = new TreeSet<Integer>();
    	for (int i = 1; i*i <= (A + B); i++) {
    		if((A+B) % i != 0)	continue;
    		wk = (A+B) / i;
    		if(wk == A || wk == B)	continue;
    		set.add(wk);
    		set.add(i);
    	}
    	int  ans = -1;
    	for(int C : set) {
			if((A+B) % C == 0 && (B+C) % A == 0 && (A+C) % B == 0) {
				ans = C;
//				out.println("条件に一致した　　ans = " + C);
			}
    	}
    	out.println(ans);
    }
    // ==================================================================
}