package main

import (
	"fmt"
)

type pair struct {
	x, y int
}

func shared(a, b pair) int {
	count := 0
	val := 0
	if a.x == b.x {
		count++
		val = a.x
	}
	if a.x == b.y {
		count++
		val = a.x
	}
	if a.y == b.x {
		count++
		val = a.y
	}
	if a.y == b.y {
		count++
		val = a.y
	}
	if count == 1 {
		return val
	}
	return 0
}

func main() {
	var n, m int
	if _, err := fmt.Scan(&n, &m); err != nil {
		return
	}

	A := make([]pair, n)
	for i := 0; i < n; i++ {
		fmt.Scan(&A[i].x, &A[i].y)
	}

	B := make([]pair, m)
	for i := 0; i < m; i++ {
		fmt.Scan(&B[i].x, &B[i].y)
	}

	VA := make(map[int]map[int]bool)
	VB := make(map[int]map[int]bool)

	for i := 0; i < n; i++ {
		VA[i] = make(map[int]bool)
	}
	for i := 0; i < m; i++ {
		VB[i] = make(map[int]bool)
	}

	for i, a := range A {
		for j, b := range B {
			s := shared(a, b)
			if s > 0 {
				VA[i][s] = true
				VB[j][s] = true
			}
		}
	}

	allShared := make(map[int]bool)

	for i, a := range A {
		for j, b := range B {
			s := shared(a, b)
			if s > 0 {
				if len(VA[i]) > 1 || len(VB[j]) > 1 {
					fmt.Println("-1")
					return
				}
				allShared[s] = true
			}
		}
	}

	if len(allShared) == 1 {
		for k := range allShared {
			fmt.Println(k)
			return
		}
	} else if len(allShared) > 1 {
		fmt.Println("0")
	} else {
		fmt.Println("-1")
	}
}