Article: Under the terms of the concluding Treaty of Versailles signed in 1919, the empire reached its greatest extent with the addition of 1,800,000 square miles (4,700,000 km2) and 13 million new subjects. The colonies of Germany and the Ottoman Empire were distributed to the Allied powers as League of Nations mandates. Britain gained control of Palestine, Transjordan, Iraq, parts of Cameroon and Togo, and Tanganyika. The Dominions themselves also acquired mandates of their own: the Union of South Africa gained South-West Africa (modern-day Namibia), Australia gained German New Guinea, and New Zealand Western Samoa. Nauru was made a combined mandate of Britain and the two Pacific Dominions.

Question: How many square miles did the Treaty of Versailles add to the British Empire?
1,800,000