Answer the following question: Given the following passage  "In 1979, the U.S. placed Libya on its list of "State Sponsors of Terrorism", while at the end of the year a demonstration torched the U.S. embassy in Tripoli in solidarity with the perpetrators of the Iran hostage crisis. The following year, Libyan fighters began intercepting U.S. fighter jets flying over the Mediterranean, signalling the collapse of relations between the two countries. Libyan relations with Lebanon and Shi'ite communities across the world also deteriorated due to the August 1978 disappearance of imam Musa al-Sadr when visiting Libya; the Lebanese accused Gaddafi of having him killed or imprisoned, a charge he denied. Relations with Syria improved, as Gaddafi and Syrian President Hafez al-Assad shared an enmity with Israel and Egypt's Sadat. In 1980, they proposed a political union, with Libya paying off Syria's £1 billion debt to the Soviet Union; although pressures led Assad to pull out, they remained allies. Another key ally was Uganda, and in 1979, Gaddafi sent 2,500 troops into Uganda to defend the regime of President Idi Amin from Tanzanian invaders. The mission failed; 400 Libyans were killed and they were forced to retreat. Gaddafi later came to regret his alliance with Amin, openly criticising him.",  answer the following question. Note that the answer is present within the text.  Question: How many Libyans were not killed when defending Amin in Uganda?
Answer:
2,500