// trees                  !!!!!! DONE !!!!!!!
#include <iostream.h>
#include <fstream.h>

class Node
{
private:
 int num;
 Node * back;

public:
 Node (int _num) {num = _num; cnt = -1; visited = 0; back = NULL;};

 void setBack (Node * _back) {back = _back;};
 Node * getBack () {return back;};

 int setChild (Node * child) {cnt++; if (cnt > 99) return 0; children[cnt] = child; return 1;};

 int bad () {return (cnt > 99);};

 int visited;   // times visited; for DFS
 int cnt;
 Node * children[10];

 int id () {return num;};

};

Node * links[20000];

int DFS (Node * cur, int & total)
{
 cur->visited++;
 if (cur->visited > 1) return 0;   // bad
 total++;

 int temp = 1;
 for (int i=0; i <= cur->cnt; i++)
  temp = temp && DFS(cur->children[i], total);

 return temp;
}

int Bad ()
{
 int node;
 int total = 0;

 // find total # of nodes & input overflow
 for (int i=0; i < 20000; i++)
  if (links[i] != NULL)
  {
   node = i;
   total++;
   if ((links[i])->bad())
    return -1;
  }

 // back up to head
 i = 0;
 Node * cur = links[node];
 while (cur->getBack() != NULL && i < total)
 {
  cur = cur->getBack();
  i++;
 }

 if (i == total) return -2;

 // DFS
 int _total = 0;
 if (!DFS(cur, _total)) return -3;
 if (_total < total) return -4;

 return cur->id();

}

void main()
{
  ifstream in;
  in.open("fight.in");
  ofstream out;
  out.open("fight.out");

  int total;

  // reset
  total = 0;

  for (int i=0; i < 20000; i++)
   links[i] = NULL;

  while (1)
  {
   int a, b;
   in >> a >> b;
   if (a < 0) break;

   if (a == 0 && b == 0)
   {
     // analyze
     int temp;
     if ((temp = Bad()) < 0)
      out << "INVALID" << endl; //Bad tree; reason # " << temp << endl;
     else
      out << temp << endl; //"Good tree; head = " << temp << endl;

     // reset
     total = 0;

     for (i=0; i < 20000; i++)
      links[i] = NULL;
   }
   else
   {
     if (links[a] == NULL)
      links[a] = new Node(a);
     if (links[b] == NULL)
      links[b] = new Node(b);

     if ((links[a])->setChild(links[b]))
     {
       (links[b])->setBack(links[a]);
     }

   }

  }

  in.close();
  out.close();
}
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