#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return true; } return false; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return true; } return false; }
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define mp make_pair
#define si(x) int(x.size())
const int mod=998244353,MAX=300005,INF=1<<30;

map<vector<ll>,ll> MA;

ll solve(vector<ll> S){
    sort(all(S));
    if(MA.count(S)) return MA[S];
    if(si(S)==1) return S[0];
    
    ll res=-1;
    for(int i=0;i<si(S);i++){
        for(int j=i+1;j<si(S);j++){
            if((S[i]+S[j])%2==0){
                vector<ll> T;
                T.push_back((S[i]+S[j])/2);
                for(int k=0;k<si(S);k++){
                    if(i==k||j==k) continue;
                    T.push_back(S[k]);
                }
                chmax(res,solve(T));
            }
        }
    }
    return MA[S]=res;
}

int main(){
    
    std::ifstream in("text.txt");
    std::cin.rdbuf(in.rdbuf());
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    int L,R;cin>>L>>R;
    if(L==R) cout<<L<<endl;
    else if(L+1==R) cout<<-1<<endl;
    else cout<<R-1<<endl;
    
    return 0;
    
    for(ll N=1;N<=10;N++){
        vector<ll> S(N);
        for(int i=0;i<N;i++) S[i]=i+1;
        cout<<solve(S)<<",";
    }
}