import sys

def main():
    N = int(input())
    tmp = input().split()
    if len(tmp) > 1:
        print('"assert"')
        return

    A = [int(tmp[0])]
    for _ in range(N-1):
        A.append(int(input()))

    A.sort(reverse=True)

    mx = A[0] + A[1]
    ans = -1
    for i in range(0, N-1):
        for j in range(i+1, N):
            if A[i] + A[j] != mx:
                ans = A[i] + A[j]
                break
        else:
            continue
        break

    print(ans)



if __name__ == "__main__":
    main()