Simplify the following text for a Grade 9 native English-speaking person.
Original: Robert Anthony Eden , 1st Earl of Avon , KG , MC , PC ( 12 June 1897 â `` 14 January 1977 ) was a British Conservative politician , who was Prime Minister from 1955 to 1957 .
Simplified: Robert Anthony Eden , 1st Earl of Avon ( 12 June 1897-14 January 1977 ) was a prime minister of the United Kingdom .