How to compare two nested dictionaries with the same keys and update values with a condition in python?

Question

d1 = {\'Berlin\': {\'Boston\' : 9, \'LA\' : 7, \'Chicago\' : 1},
      \'Vienna\': {\'Boston\' : 5, \'LA\' : 2, \'Chicago\' : 8}, 
      \'London\': {\'Boston\' : 8,