def calc7(s):
#    for i in s:
#        if s[i] == 7:
#         ret = s.sub(s[i+1],s[i+1]*2)
#         i += 1
#         return ret
#        ret = sum(s)
#    else:
#        ret = sum(s)
    if len(s) == 0:
        ret = -1
    else:
        ret = sum(s)
    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([]))