#include <bits/stdc++.h>
using namespace std;
/*{{{*/  //template
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define rrep(i,n) for(int i=(n)-1;i>=0;i--)
constexpr int INF = numeric_limits<int>::max()/2;
constexpr long long LINF = numeric_limits<long long>::max()/3;
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define all(v) (v).begin(),(v).end()
#define sz(x) (int)(x).size()
#define debug(x) cerr<<#x<<":"<<x<<endl
#define debug2(x,y) cerr<<#x<<","<<#y":"<<x<<","<<y<<endl
//struct fin{ fin(){ cin.tie(0); ios::sync_with_stdio(false); } } fin_;
struct Double{ double d; explicit Double(double x) : d(x){} };
ostream& operator<<(ostream& os,const Double x){ os << fixed << setprecision(20) << x.d; return os; }
template<typename T> ostream& operator<<(ostream& os,const vector<T>& vec){ os << "["; for(const auto& v : vec){ os << v << ","; } os << "]"; return os; }
template<typename T,typename U> ostream& operator<<(ostream& os, const map<T,U>& mp){ os << "{"; for(auto& p : mp){ os << p << ","; } os << "}"; return os; }
template<typename T,typename U> ostream& operator<<(ostream& os,const pair<T,U>& p){ os << "(" << p.first << ","<< p.second <<")"; return os; }
template<typename T> ostream& operator<<(ostream& os,const set<T>& st){ os<<"{"; for(T v:st) os<<v<<","; os <<"}"; return os; }
template<typename T,typename U> inline void chmax(T &x,U y){ if(y>x) x = y; }
template<typename T,typename U> inline void chmin(T &x,U y){ if(y<x) x = y; }
using ll = long long;
using ull = unsigned long long;
using pii = std::pair<int,int>;
using vi = std::vector<int>;
typedef vector<vi> vvi;
ll gcd(ll a,ll b){ if(b==0) return a; else return gcd(b,a%b); }
constexpr ll TEN(int n) { return (n == 0) ? 1 : 10 * TEN(n - 1); }
//constexpr double eps = 1e-14; 
constexpr double eps = 1e-10; 
constexpr ll mod = 1e9+7;
const int dx[]={1,0,-1,0} ,dy[] = {0,1,0,-1};
/*}}}*/

void solve(int n,int m){
    vector<ll> a(n);
    vector<ll> w(m);
    rep(i,n) cin >> a[i];
    rep(i,m) cin >> w[i];
    sort(all(a));

    unordered_set<ll> val; // 3 ^ 10 =  6 * 10 ^ 4
    for(int mask1=0;mask1<(1<<m);mask1++){
        for(int mask2=0;mask2<(1<<m);mask2++){
            if(mask1 & mask2) continue;
            ll sum = 0;
            for(int i=0;i<m;i++){
                int m1 = (mask1 & (1<<i));
                int m2 = (mask2 & (1<<i));
                if(m1 and !m2){
                    sum += w[i];
                }else if(!m1 and m2){
                    sum -= w[i];
                }
            }

            val.insert(sum);
        }
    }

    bool ok = true;
    for(int i=0;i<n;i++){
        if(!val.count(a[i])){
            ok = false;
            break;
        }
    }
    if(ok){
        cout << 0 << endl;
        return;
    }

    set<ll> cand;
    for(int i=0;i<n;i++){
        if(val.count(a[i])) continue;

        for(auto v : val){
            if(v!=a[i]) cand.insert(abs(v-a[i]));
        }

        break;
    }

    for(auto c : cand){
        bool ok = true;
        for(int i=0;i<n;i++){
            if(!(val.count(a[i]) || val.count(a[i]-c) || val.count(a[i]+c))){
                ok = false;
                break;
            }
        }
        if(ok){
            cout << c << endl;
            return;
        }
    }
    cout << -1 << endl;
}

int main(){
    while(1){
        int n,m; cin >> n >> m;
        if(n == 0 and m == 0) break;
        solve(n,m);
    }
}



Measured memory_bytes: 8064