President Trump just presented Mariano Rivera with the Presidential Medal of Freedom on Monday ... and unfortunately, he left his calculator behind.
POTUS -- a HUGE Yankees fan -- honored the Sandman at 1600 Penn on Monday ... praising "maybe the greatest pitcher of all time" for his accomplishments on and off the mound.
FYI-- the Presidential Medal of Freedom is the highest award a civilian can achieve in the United States -- and is given to people who make an especially great contribution to American society.
But the funny part is when Trump tried to do some impromptu math .