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>
using namespace std;
const int MAXN = 5e5 + 5;
template <typename T>
void chkmax(T& x, T y) {
  if (y > x) x = y;
}
template <typename T>
void chkmin(T& x, T y) {
  if (y < x) x = y;
}
int N, M;
struct node {
  int x, y;
} a[MAXN], b[MAXN];
int main() {
  scanf("%d%d", &N, &M);
  for (int i = 1; i <= N; ++i) {
    scanf("%d%d", &a[i].x, &a[i].y);
    if (a[i].x > a[i].y) swap(a[i].x, a[i].y);
  }
  for (int i = 1; i <= M; ++i) {
    scanf("%d%d", &b[i].x, &b[i].y);
    if (b[i].x > b[i].y) swap(b[i].x, b[i].y);
  }
  int per1 = 0;
  for (int i = 1; i <= N; ++i) {
    int cur = 0;
    for (int j = 1; j <= M; ++j) {
      if (a[i].x == b[j].x && a[i].y == b[j].y) continue;
      int qwq;
      if (a[i].x == b[j].x || a[i].x == b[j].y)
        qwq = a[i].x;
      else if (a[i].y == b[j].x || a[i].y == b[j].y)
        qwq = a[i].y;
      else
        continue;
      if (!cur)
        cur = qwq;
      else if (cur != qwq) {
        per1 = -1;
        break;
      }
    }
    if (per1 == -1) break;
    if (cur) per1 = 1;
  }
  int per2 = 0;
  for (int i = 1; i <= M; ++i) {
    int cur = 0;
    for (int j = 1; j <= N; ++j) {
      if (a[i].x == b[j].x && a[i].y == b[j].y) continue;
      int qwq;
      if (a[i].x == b[j].x || a[i].x == b[j].y)
        qwq = a[i].x;
      else if (a[i].y == b[j].x || a[i].y == b[j].y)
        qwq = a[i].y;
      else
        continue;
      if (!cur)
        cur = qwq;
      else if (cur != qwq) {
        per2 = -1;
        break;
      }
    }
    if (per2 == -1) break;
    if (cur) per2 = 1;
  }
  int my = 0;
  for (int i = 1; i <= N; ++i) {
    for (int j = 1; j <= M; ++j) {
      if (a[i].x == b[j].x && a[i].y == b[j].y) continue;
      int qwq;
      if (a[i].x == b[j].x || a[i].x == b[j].y)
        qwq = a[i].x;
      else if (a[i].y == b[j].x || a[i].y == b[j].y)
        qwq = a[i].y;
      else
        continue;
      if (!my)
        my = qwq;
      else if (my != qwq) {
        my = -1;
        break;
      }
    }
    if (my == -1) break;
  }
  if (my >= 1)
    printf("%d", my);
  else if (per1 == 1 && per2 == 1)
    printf("0");
  else
    printf("-1");
  return 0;
}
