The Federal Reserve System (also known as the Federal Reserve or simply the Fed) is the central banking system of the United States. It was created on December 23, 1913, with the enactment of the Federal Reserve Act, after a series of financial panics (particularly the panic of 1907) led to the desire for central control of the monetary system in order to alleviate financial crises. Over the years, events such as the Great Depression in the 1930s and the Great Recession during the 2000s have led to the expansion of the roles and responsibilities of the Federal Reserve System.
The U.S. Constitution provides that Congress shall have the power to "borrow money on the credit of the United States". Congress has exercised that power by authorizing Federal Reserve Banks to issue Federal Reserve Notes. Those notes are "obligations of the United States" and "shall be redeemed in lawful money on demand at the Treasury Department of the United States, in the city of Washington, District of Columbia, or at any Federal Reserve bank". Federal Reserve Notes are designated by law as "legal tender" for the payment of debts. Congress has also authorized the issuance of more than 10 other types of banknotes, including the United States Note and the Federal Reserve Bank Note. The Federal Reserve Note is the only type that remains in circulation since the 1970s.
Hafizullah Amin (Pashto/Dari: حفيظ الله امين‎; born 1 August 1929 – 27 December 1979) was an Afghan communist politician during the Cold War. Amin was born in Paghman and educated at Kabul University, after which he started his career as a teacher. After a few years in that occupation, he went to the United States to study. He would visit the United States a second time before moving permanently to Afghanistan, and starting his career in radical politics. He ran as a candidate in the 1965 parliamentary election but failed to secure a seat. Amin was the only Khalqist elected to parliament in the 1969 parliamentary election, thus increasing his standing within the party. He was one of the leading organizers of the Saur Revolution which overthrew the government of Mohammad Daoud Khan. In 1979 he named himself president, prime minister, and chairman of the Khalq wing. He has been described as "ruthless" and a "radical Marxist".Amin's short-lived presidency was marked by controversies from beginning to end. He came to power by disposing of his predecessor Nur Muhammad Taraki and later ordering his death. Amin made attempts to win support from those who revolted against the communist regime which had begun under Taraki, but his government was unable to solve this problem. Many Afghans held Amin responsible for the regime's harshest measures, such as ordering thousands of executions. Thousands of people disappeared without trace during his time in office. The Soviet Union, which was dissatisfied with Amin, intervened in Afghanistan while invoking the Twenty-Year Treaty of Friendship between Afghanistan and the Soviet Union. Amin was assassinated by the Soviets on December 27, 1979 as part of Operation Storm-333, having ruled for slightly longer than three months.

Question: What are the notes issued by the current central bank of the country Amin visited twice?
Answer: obligations of the United States