#include <bits/stdc++.h>
using namespace std;
int n, m;
vector<pair<int, int> > a_sent;
vector<pair<int, int> > b_sent;
set<pair<int, int> > a_possible;
set<pair<int, int> > b_possible;
int vis[10];
bool certain(pair<int, int> a, vector<pair<int, int> > sent) {
  bool can_x = false;
  bool can_y = false;
  for (pair<int, int> b : sent) {
    if (0) {
      printf("has: [%d, %d], try: [%d, %d]\n", a.first, a.second, b.first,
             b.second);
    }
    if (a.first == b.first && a.second == b.second) continue;
    if (a.first == b.second && a.second == b.first) continue;
    if (a.first != b.first && a.second != b.second && a.first != b.second &&
        a.second != b.first)
      continue;
    if (a.first == b.first || a.first == b.second) can_x = true;
    if (a.second == b.first || a.second == b.second) can_y = true;
  }
  if (0) printf("can_x: %d, can_y: %d\n", can_x, can_y);
  return (can_x && !can_y) || (can_y && !can_x);
}
int main() {
  cin >> n >> m;
  while (n--) {
    pair<int, int> pair;
    cin >> pair.first >> pair.second;
    if (pair.first < pair.second) swap(pair.first, pair.second);
    a_sent.push_back(pair);
  }
  while (m--) {
    pair<int, int> pair;
    cin >> pair.first >> pair.second;
    if (pair.first < pair.second) swap(pair.first, pair.second);
    b_sent.push_back(pair);
  }
  for (pair<int, int> a : a_sent) {
    for (pair<int, int> b : b_sent) {
      if (a.first == b.first && a.second == b.second) continue;
      if (a.first == b.second && a.second == b.first) continue;
      if (a.first != b.first && a.second != b.second && a.first != b.second &&
          a.second != b.first)
        continue;
      if (a.first == b.first) vis[a.first]++;
      if (a.second == b.second) vis[a.second]++;
      if (a.first == b.second) vis[a.first]++;
      if (a.second == b.first) vis[a.second]++;
      a_possible.insert(a);
      b_possible.insert(b);
    }
  }
  int cnt = 0;
  for (int i = 1; i <= 9; ++i) {
    cnt += vis[i] >= 1;
    if (0) printf("vis[%d] = %d\n", i, vis[i]);
  }
  if (0) cout << cnt << endl;
  if (cnt == 1) {
    for (int i = 1; i <= 9; ++i) {
      if (vis[i] >= 1) {
        cout << i << endl;
        return 0;
      }
    }
  }
  for (pair<int, int> a : a_possible) {
    if (!certain(a, b_sent)) {
      cout << -1 << endl;
      return 0;
    }
  }
  for (pair<int, int> b : b_possible) {
    if (!certain(b, a_sent)) {
      cout << -1 << endl;
      return 0;
    }
  }
  cout << 0 << endl;
}
