def calc7(l):
    if len(l) == 0:
        return -1
    sum = 0
    for i, j in enumerate(l):
        if j == 7 and i < len(l)-1:
            l[i+1] *= 2
        sum += j
    return sum








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