20. Shrek, the ogre, went through great hardships to rescue Princess Fiona, who was trapped in a high tower. Fiona asked Shrek: “How old are you this year?” Shrek replied: “My age 13 years from now is an integer multiple of my age 37 years ago.” The clever Princess Fiona thought for a moment and said: “You are at most $\qquad$ years old this year.”
$87$