def calc7(arr):
    if len(arr) == 0:
        return -1
    else:
        is_7 = False
        ret = 0
        for i in arr:
            if is_7:
                ret += i * 2
                is_7 = False
            else:
                ret += i
                if i == 7:
                    is_7 = True
        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([]))