/*

     题意：告诉一个区间[L,R]，问根节点的n是多少

     DFS+剪枝：父亲节点有四种情况：[l, r + len],[l, r + len - 1],[l - len, r],[l - len -1,r];

 */

 #include <cstdio>

 #include <algorithm>

 #include <cstring>

 #include <cmath>

 #include <queue>

 using namespace std;

 typedef long long ll;

 const int INF = 0x3f3f3f3f;

 const ll INFL = 1e30;

 ll ans;

 void DFS(ll l, ll r)  {

     if (l > r || l < ) return ;

     if (l == ) {

         ans = min (ans, r); return ;

     }

     ll len = r - l + ;

     if (l < len)    return ;

     DFS (l, r + len);

     if (len != )   DFS (l, r + len - );

     DFS (l - len, r);   DFS (l - len - , r);

 }

 int main(void)  {       //HDOJ 5323 Solve this interesting problem

     //freopen ("H.in", "r", stdin);

     ll L, R;

     while (scanf ("%I64d%I64d", &L, &R) == ) {

         if (L == )   {

             if (R >= L) printf ("%I64d\n", R);

             else    puts ("-1");

             continue;

         }

         ans = INFL; DFS (L, R);

         printf ("%I64d\n", (ans == INFL) ? - : ans);

     }

     return ;

 }