Article: Each year, nearly $200 million in hunters' federal excise taxes are distributed to state agencies to support wildlife management programs, the purchase of lands open to hunters, and hunter education and safety classes. Since 1934, the sale of Federal Duck Stamps, a required purchase for migratory waterfowl hunters over sixteen years old, has raised over $700 million to help purchase more than 5,200,000 acres (8,100 sq mi; 21,000 km2) of habitat for the National Wildlife Refuge System lands that support waterfowl and many other wildlife species and are often open to hunting. States also collect money from hunting licenses to assist with management of game animals, as designated by law. A key task of federal and state park rangers and game wardens is to enforce laws and regulations related to hunting, including species protection, hunting seasons, and hunting bans.

Question: What does land has Federal Duck Stamp money helped purchase?
5,200,000 acres