#include<bits/stdc++.h>

using namespace std;


typedef long long ll;
typedef long double ld;

#define endl "\n"
#define all(a) a.begin(), a.end()
#define pb push_back
#define eb emplace_back
//#define int long long



// Don't Start typing till you complete the idea.


// Check these things for WA and before submitting
// 1. Did you clear all the global arrays
// 2. Did you checked your <= >= < > 
// 3. Did you take the input properly. Did you use break or return while taking input?
// 4. Did you divide by 0.
// 5. Check array , vector , etc size
// 6. in the while loop did you miss something that would cause infinite loop?
// 7. Did you save your dp?
// 8. Are you trying to use something after deleting it?
// 9. Did you read the problem statement wrong?
// 10. Did you check the constrains of the problem properly
// 11. Did you checked for smaller cases like 1 , 0 , etc
// 12. Is it something to with overflow?
// 13. Did you initialized all the variables properly?
// 14. Did you use the formulas correctly?

// STRESS TESTING !!!!!! STRESS TESTING !!!!!
// STRESS Testing Not working?
// Stress test for multiple cases? 
// Stress test for big inputs?
// Stress test for really small inputs?
// Even then if it doesn't work -> Did you wrote the wrong Brute force code


// Check these things if you are not generating ideas
// 1. Did you try thinking it in reverse?
// 2. Read the problem statement again
// 3. Check the constraints again
// 4. Try smaller cases in notebook and try to expand
// 5. Think about invariants
// 6. Think simpler ideas maybe?
// 7. Think brute force and try to get something out of it.
// 8. Maybe take a deep breath and take a break for few minutes and drink some water? :3 

void GG()
{
    int l , r;
    cin >> l >> r;
    if(r - l == 0){
        cout << l << endl;
    }
    else if(r - l == 1){
        cout << -1 << endl;
    }
    else {
        cout << r - 1 << endl;
    }
}

signed main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(0);
	cout.tie(0);

	int ttc=1;
	// cin>>ttc;
	//int cnt=0;
	while(ttc--)
	{
		//cout<<"Case "<<++cnt<<": ";
		GG();
	}
}