Before you watch the video, make sure you know these words:

a ledger = a book in which a company or organization writes down the amounts of money it spends and receives
to tamper with= to interfere with (something) in order to cause damage or make unauthorized alterations
valid= legally or officially acceptable
a hash= a mix of data (here)
a node = a computer in a network of computers
a P2P network= a peer-to-peer network = a decentralized net of nodes(computers) equal in rights 
a notary= a person authorized to perform certain legal formalities, especially to draw up or certify contracts, deeds, and other documents for use in other jurisdictions
a tax = an amount of money that you have to pay to the government so that it can pay for public services.

I. Watch the video and fill in the gaps in the text below with suitable words.

A block chain is a register of some digital documents organized in such a way that it is almost impossible either to ______ them, or to tamper with them. 
The three main elements of a block in a block chain are some ________, some hash and the hash of the previous block.
A bitcoin block contains typically data on the sender, ________ and the amount of coins.
Just like a fingerprint, hash is _________ and identifies the block with its data. If it changes, the block itself is changed.
The genesis block cannot point to any previous block.
Changing a single block will make all the consequent blocks_________. But modern computers are efficient enough to recalculate the hashes of all the following chain once you change the hash of one particular block, making the tampered block and all the others after it valid again.
How is tampering prevented? Proof-of-work is done on each newly created block, which takes time.
Besides, instead of using a central managing entity, block chains reside on P2P networks, which means every member of the networks, called a____________, has a full copy of the block chain and can use it to verify that the block chain remains unmodified.
SO, when someone creates a new block, that block is sent to everyone on the network. Each node verifies the block to make sure it is unique and hasn’t been created from one of the existing ones. If everything checks out, the new block is added to the block chain of each node.
In other words, nodes on the network have to achieve consensus which blocks are valid and which have been _____with.
Block chains are constantly __________. One of the latest developments is the creation of ________contracts, which are simple programs that code for automatic transactions based on certain _________, like coin exchange for example.
Some other uses of block chains could be keeping medical records, notaries or even collecting _____.

II. Mute the video, turn on Russian captions and try to translate them into English orally, keeping pace with the video.