#!/usr/bin/python3

def calc7(a):
    if (len(a) is  0):
        return -1
    lucky = False
    ret = 0
    for e in a:
        if lucky:
            ret += (e * 2)
        else:
            ret += e
        if e is 7:
            lucky = True
        else:
            lucky = False
    return ret

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