def solution(a):
    rd = dict()
    la = len(a)
    if la == 0:
        return -1
    for v in a:
        if v in rd:
            rd[v] = rd[v] + 1
        else:
            rd[v] = 1
    dom = max(rd.values())
    if dom > la / 2:
        return a.index(list(rd.keys())[list(rd.values()).index(max(rd.values()))])
    else:
        return -1



a = [3, 4, 3, 2, 3, -1, 3, 3]
print(solution(a))