Hey Mohamed, welcome to Back yard chickens, I also have lost a few of my chickens to the neighbor's dogs. I am sorry for your loss, maybe these folks could recommend some security measures for the rest of your flock....
Hello, Mohamed, and welcome to BYC! Glad you joined. Good luck keeping your flock safe. I would save up for an electric fence. That is what I use and I've never lost a bird since confining them behind the electric fence.
So sorry for your losses. Definitely either use a secure run or install some electric fencing, its about all you can do. Good luck with your flock and welcome to ours!