#include <iostream>
#include <vector>


int main(void){
    std::cin.tie(nullptr); std::cin.sync_with_stdio(false);
    int n; std::cin >> n; n -= 2;
    long long int a, b; std::cin >> a >> b;
    long long int B = 0; while(n--){ long long int x; std::cin >> x; B ^= x; }

    const long long int A = a + b;
    if(not ((A >= B) and (A - B) % 2 == 0)){ std::cout << "-1\n"; return 0; }

    long long int C = (A - B) / 2;
    //if(B & C){ std::cout << "-1\n"; return 0; }
    long long int res = 0;
    std::vector<long long int> wait(0);
    for(int i = 60; i >= 0; --i){
        const bool flagAnd = (C >> i) & 1;
        const bool flagXor = (B >> i) & 1;
        if(flagAnd){
            if(flagXor){ std::cout << "-1\n"; return 0; }
            res |= (1LL << i);
            if(res > a){ std::cout << "-1\n"; return 0; }
        }
        else{
            if(flagXor) wait.push_back(1LL << i);
        }
    }
    for(const long long int x : wait) if((res | x) <= a) res |= x;
    if(res == 0){ std::cout << "-1\n"; return 0; }
    std::cout << (a - res) << '\n';

    return 0;
}
