def calc7(vals):
    calk = 0
    luckky = 1

    for val in vals:
        calk += val * luckky
        if val == 7:
            luckky = 2
        else:
            luckky = 1

    if calk == 0:
        calk = -1

    return calk


print(calc7([1, 2]))
print(calc7([3, 7]))
print(calc7([7, 5, 6]))
print(calc7([7, 9, 7, 9, 7, 9]))
print(calc7([]))