#include<bits/stdc++.h>

using namespace std;
typedef long long ll;

typedef pair<int, int> pii;
typedef vector<int> vi;
#define S(a) scanf("%d",&a)
#define LS(a) scanf("%lld",&a)
#define D(a) scanf("%lf",&a)
#define LD(a) scanf("%lf",&a)
#define FOR(i, a, b) for(int i = a;i <= b;++i)
#define DOW(i, b, a) for(int i = b; i >= a;--i)
#define Sort(a) sort(a.begin(),a.end())
#define eb emplace_back
#define fi first
#define se second


const ld DINF = 1e45;
const ll INF = 1e17;
const ll MOD = 1e9 + 7;
const int MAXF = 500010;


int getCommon(pair<int, int> a, pair<int, int> b) {

    if (a.first == b.first) return a.first;
    if (a.second == b.first) return a.second;
    if (a.first == b.second) return a.first;
    if (a.second == b.second) return a.second;
    return -1;
}

vector<int> check(vector<pair<int, int>> a, vector<pair<int, int>> b) {
    set<int> com;
    for (int i = 0; i < a.size(); ++i) {
        set<int> cme;
        for (int j = 0; j < b.size(); ++j) {
            if (a[i] != b[j]) {
                int cm = getCommon(a[i], b[j]);
                if (cm != -1) {
                    cme.insert(cm);
                    if (cme.size() >= 2) return {-1};

                }
            }
        }
        if (cme.size() > 0)
            com.insert(*cme.begin());
    }
    vector<int> ans(com.begin(), com.end());
    return ans;
}

int main() {
   // freopen("input.txt", "r", stdin);
    //  freopen("output.txt","w",stdout);
    int t;
    //S(t);
     t = 1;
    for (int tc = 1; tc <= t; ++tc) {
         int n,m;
        S(n),S(m);
        vector<pair<int, int>> a(n),b(m);
        for (int i = 0; i < n ; ++i) {
            S(a[i].first);
            S(a[i].second);
            int x = max(a[i].first,a[i].second);
            int y = min(a[i].first,a[i].second);
            a[i] = {x,y};
        }
        for (int j = 0; j < m ; ++j) {
            S(b[j].first);
            S(b[j].second);
            int x = max(b[j].first,b[j].second);
            int y = min(b[j].first,b[j].second);
            b[j] = {x,y};
        }
        vector<int> res1 = check(a,b);
        vector<int> res2 = check(b,a);
        if(res1.size() == 0 || res2.size() == 0 ||res1[0] == -1 || res2[0] == -1 || res1 != res2) {
            printf("-1\n");
        }else if(res1 == res2 && res1.size() >= 2){
            printf("0\n");
        }else {
            printf("%d\n",res1[0]);
        }


    }

    return 0;
}