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# you can write to stdout for debugging purposes, e.g.
# print("this is a debug message")

def solution(A):
    # write your code in Python 3.6
    if A == []:
        return -1
    dominator = [0, A[0], 1]
    for i in range(1, len(A)):
        if A[i] == dominator[1]:
            dominator[2] += 1
        elif dominator[2] == 0:
            dominator[1] = A[i]
            dominator[2] = 1
            dominator[0] = i
        else:
            dominator[2] -= 1
    if A.count(dominator[1]) <= len(A) / 2:
        return -1
    return dominator[0]