题解 | 预知

T = int(input())
for t in range(T):
    n = int(input())
    arr = list(map(int,input().split()))
    arr1 = arr.copy()
    arr1.remove(max(arr1))
    if len(arr1) == 0:
        print(-1)
    else:
        if max(arr1) == 1:
            print(max(arr) - 1)
        else:
            print(max(arr))