#include <bits/stdc++.h>
#define fp(i, a, b) for (int i(a), ed(b); i <= ed; ++i)
#define fb(i, a, b) for (int i(a), ed(b); i >= ed; --i)
#define go(u, i) for (int i(head[u]); i; i = e[i].nxt)
using namespace std;
typedef long long Int;
typedef pair<int, int> pii;
////////////////////////////////////////////////////////////////////////////////////////////////////
template <typename T1, typename T2>
inline void chkmin(T1& a, T2 b) {
    if (a > b) a = b;
}
template <typename T1, typename T2>
inline void chkmax(T1& a, T2 b) {
    if (a < b) a = b;
}
namespace FastIO {
char buf[1 << 21], *p1 = buf, *p2 = buf;
#define getchar() (p1 == p2 && (p1 = buf, p2 = (p1 + fread(buf, 1, 1 << 21, stdin))) == p1 ? EOF : *p1++)
template <typename T>
inline T read() {
    T x = 0, w = 0;
    char ch = getchar();
    while (ch < '0' || ch > '9') w |= (ch == '-'), ch = getchar();
    while ('0' <= ch && ch <= '9') x = x * 10 + (ch ^ '0'), ch = getchar();
    return w ? -x : x;
}
template <typename T>
inline void write(T x) {
    if (!x) return;
    write<T>(x / 10), putchar((x % 10) ^ '0');
}
template <typename T>
inline void print(T x) {
    if (x > 0)
        write<T>(x);
    else if (x < 0)
        putchar('-'), write<T>(-x);
    else
        putchar('0');
}
template <typename T>
inline void print(T x, char en) {
    print<T>(x), putchar(en);
}
};  // namespace FastIO
using namespace FastIO;
#undef getchar()
const int maxn = 2e5 + 10, maxm = 20, mod = 998244353;
int n, b[maxn];
int main() {
    n = FastIO::read<int>();
    fp(i, 1, n) b[i] = FastIO::read<int>();
    int ans = 0;
    fp(i, 1, n) ans |= b[i];
    if (!ans) {
        ans = 1;
        while (1) {
            ans *= 2;
            if (ans > n) break;
        }
    }
    fp(i, 1, n) {
        if ((i & ans) != b[i]) {
            ans = -1;
            break;
        }
    }
    cout << ans << "\n";
    return 0;
}