2 3
1 2 1 3
1 2 1 3 2 3
 3 7
8 2 7 9 8 1
3 1 8 1 2 7 4 7 4 2 1 4 4 6
 1 10
3 9
3 2 3 4 5 3 5 7 8 6 2 5 7 8 2 4 1 7 5 1
 12 1
6 2 6 4 8 6 6 9 5 6 6 1 9 1 1 3 3 9 2 4 5 2 8 1
6 7
 2 3
1 2 7 8
1 3 2 4 7 9
 3 2
1 2 1 3 2 3
1 2 4 5
 3 8
8 9 8 5 9 2
8 4 8 3 2 6 4 2 4 3 3 7 3 6 1 6
 8 5
7 9 6 7 4 7 2 1 4 9 2 9 4 2 9 6
8 7 1 8 8 5 3 5 3 8
 4 4
1 2 1 3 6 7 6 8
1 4 1 5 6 1 6 9
 3 10
6 1 4 1 4 6
7 1 8 1 8 5 3 2 9 7 9 3 5 9 5 3 5 7 7 2
 3 3
1 2 1 3 2 3
1 2 1 3 2 3
 11 1
2 6 1 4 7 9 7 6 8 1 4 8 4 7 7 2 1 7 9 6 6 5
3 1
 5 6
1 7 5 6 6 9 3 6 1 9
2 7 2 5 8 5 4 8 4 2 8 2
 8 1
1 6 7 6 7 3 9 2 1 2 8 6 2 3 4 1
8 3
 9 4
2 8 8 9 8 1 9 2 5 9 3 5 3 2 5 2 9 1
8 4 8 7 6 8 4 7
 4 7
9 2 4 1 2 3 2 7
6 1 5 4 7 5 6 3 1 5 8 1 1 4
 3 3
1 2 5 6 7 8
2 3 4 5 8 9
 1 7
8 4
9 8 8 2 6 8 8 1 7 8 2 1 9 5
 2 2
1 2 2 3
2 3 3 4
 12 12
6 7 5 4 7 8 2 9 8 5 3 5 1 6 7 3 7 9 5 7 1 8 6 8
6 4 2 1 7 8 1 6 8 5 9 8 1 5 7 2 5 9 6 3 9 2 9 4
 9 1
3 4 3 2 3 7 3 5 9 4 1 9 6 4 5 2 7 6
8 3
 9 2
9 6 1 6 2 5 7 3 8 1 7 2 9 1 2 8 3 8
6 4 4 5
 3 9
9 7 9 2 7 2
9 8 1 9 3 9 6 3 8 6 4 6 1 3 5 4 5 3
 3 6
3 5 7 4 7 5
3 9 3 2 8 6 6 2 8 2 6 9
 2 1
1 2 1 3
1 2
 9 1
1 8 7 6 7 2 7 9 4 1 4 3 3 8 4 6 9 6
9 4
 2 2
1 2 2 4
1 2 1 3
 6 9
8 1 8 4 2 8 2 1 4 1 4 2
8 3 8 6 7 8 5 8 6 7 5 7 9 6 5 6 5 3
 12 2
3 1 8 2 6 9 2 6 5 4 4 3 4 1 4 2 6 3 9 7 9 4 3 2
7 1 4 1
 2 1
4 5 6 7
4 7
 4 3
1 2 4 5 6 7 8 9
1 2 8 9 3 1
 12 12
7 6 3 8 8 4 4 7 1 9 9 5 7 5 4 9 8 6 2 7 7 3 3 6
9 1 2 4 9 8 5 3 6 7 3 8 2 7 5 9 6 4 3 1 2 6 1 4
 12 5
9 2 6 7 7 8 3 4 8 4 7 1 2 1 7 3 7 2 5 6 3 8 1 5
3 7 7 5 7 4 5 8 4 6
 7 6
6 2 9 2 6 5 2 4 1 2 4 5 6 7
3 9 5 1 9 8 9 5 3 4 2 3
 6 4
2 7 3 2 8 3 1 5 7 4 3 5
2 6 9 8 8 6 6 9
 10 2
4 9 2 1 5 1 6 2 6 7 2 7 5 8 1 7 5 3 9 1
9 7 1 4
 4 4
1 2 2 3 1 3 4 5
1 3 3 2 1 2 4 6
 3 4
2 1 8 9 3 1
1 2 4 5 6 7 8 9
 1 12
6 8
8 4 8 2 5 8 9 8 8 3 8 7 8 1 1 3 1 9 4 3 7 3 5 7
 4 4
1 2 3 4 5 6 7 8
2 3 4 5 6 7 8 1
 3 10
1 5 7 1 2 1
9 5 5 6 3 5 4 7 8 3 9 6 8 4 9 8 4 6 3 4
 3 2
1 2 4 5 6 7
4 7 1 3
 5 6
4 7 7 3 4 3 9 4 3 9
7 5 7 8 1 7 7 2 6 2 1 2
 7 3
2 6 6 7 6 4 6 1 9 6 7 4 1 9
6 5 3 6 6 8
 2 7
2 7 2 5
7 1 9 7 8 9 4 9 8 1 3 9 3 8
 2 2
1 2 1 3
1 2 1 3
 2 11
6 1 3 6
1 7 1 2 1 5 1 4 5 3 3 2 9 8 4 2 7 5 4 9 2 9
 12 12
1 6 2 6 8 3 6 4 4 8 7 2 7 5 9 4 2 4 9 5 8 5 3 6
2 8 6 9 2 6 7 4 6 5 6 3 5 8 7 8 7 1 1 9 9 7 7 3
 4 12
2 8 3 1 2 1 9 4
9 5 5 3 1 6 3 7 7 1 8 5 6 5 4 6 1 9 1 4 2 5 9 8

-1 -1 3 6 -1 -1 0 0 -1 1 -1 1 0 -1 8 -1 0 8 0 -1 3 0 9 3 1 -1 0 8 -1 -1 1 -1 -1 -1 0 -1 -1 1 8 -1 0 -1 7 6 7 1 0 -1 -1
#include <bits/stdc++.h>
const int md = 1000000007;
long long inf = 1000000000001;
using namespace std;
using namespace std::chrono;
long long pw(long long a, long long b) {
  long long c = 1, m = a;
  while (b) {
    if (b & 1) c = (c * m);
    m = (m * m);
    b /= 2;
  }
  return c;
}
long long pwmd(long long a, long long b) {
  long long c = 1, m = a;
  while (b) {
    if (b & 1) c = (c * m) % md;
    m = (m * m) % md;
    b /= 2;
  }
  return c;
}
long long modinv(long long n) { return pwmd(n, md - 2); }
long long inverse(long long i) {
  if (i == 1) return 1;
  return (md - ((md / i) * inverse(md % i)) % md + md) % md;
}
int ceel(int a, int b) {
  if (a % b == 0)
    return a / b;
  else
    return a / b + 1;
}
int my_log(int n, int b) {
  long long i = 1;
  long long ans = 0;
  while (1) {
    if (i > n) {
      ans--;
      break;
    }
    if (i == n) break;
    i *= b;
    ans++;
  }
  return ans;
}
bool sortbysec(const pair<long long, long long> &a,
               const pair<long long, long long> &b) {
  return (a.second < b.second);
}
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
vector<char> capl = {'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I',
                     'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R',
                     'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z'};
vector<char> sml = {'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i',
                    'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r',
                    's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};
string conv(long long a) {
  string b;
  int c = my_log(a, 2);
  c++;
  b = '1';
  for (int i = 0; i < c - 1; i++) {
    long long k = a >> (c - 2 - i);
    if (k & 1)
      b += '1';
    else
      b += '0';
  }
  return b;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  ;
  int n, a, m, b;
  cin >> n >> m;
  vector<pair<int, int>> vv1, vv2;
  for (int i = 0; i < n; i++)
    cin >> a >> b, vv1.push_back({min(a, b), max(a, b)});
  for (int i = 0; i < m; i++)
    cin >> a >> b, vv2.push_back({min(a, b), max(a, b)});
  vector<int> v1[n + 1];
  vector<int> v2[m + 1];
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < m; j++) {
      if (vv1[i].first == vv2[j].first || vv1[i].second == vv2[j].second ||
          vv1[i].first == vv2[j].second || vv1[i].second == vv2[j].first) {
        if (vv1[i].first == vv2[j].first && vv1[i].second == vv2[j].second)
          continue;
        else
          v1[i].push_back(j);
      }
    }
  }
  for (int i = 0; i < m; i++) {
    for (int j = 0; j < n; j++) {
      if (vv2[i].first == vv1[j].first || vv2[i].second == vv1[j].second ||
          vv2[i].first == vv1[j].second || vv1[j].first == vv2[i].second) {
        if (vv2[i].first == vv1[j].first && vv2[i].second == vv1[j].second)
          continue;
        else
          v2[i].push_back(j);
      }
    }
  }
  int flag1 = 0, flag2 = 0, cnt = 0;
  for (int i = 0; i < n; i++) {
    if (v1[i].size() > 0) {
      cnt++;
      if (v1[i].size() > 1) {
        flag2 = 1;
        break;
      }
    }
  }
  if (flag2) {
    cout << "-1";
    return 0;
  }
  for (int i = 0; i < m; i++) {
    if (v2[i].size() > 0) {
      cnt++;
      if (v2[i].size() > 1) {
        flag2 = 1;
        break;
      }
    }
  }
  if (flag2)
    cout << "-1";
  else {
    if (cnt > 2) {
      cout << "0";
      return 0;
    }
    for (int i = 0; i < n; i++) {
      if (v1[i].size() == 1) {
        int y = v1[i][0];
        if (vv1[i].first == vv2[y].first) {
          cout << vv1[i].first;
          return 0;
        } else {
          cout << vv1[i].second;
          return 0;
        }
      }
    }
    for (int i = 0; i < m; i++) {
      if (v2[i].size() == 1) {
        int y = v2[i][0];
        if (vv2[i].first == vv1[y].first) {
          cout << vv2[i].first;
          return 0;
        } else {
          cout << vv2[i].second;
          return 0;
        }
      }
    }
  }
}
