What is the difference in fair value between developed technology and customer relationships?
{"": {"Fair Value": "(U.S. $ in thousands)", "Useful Life": "(years)"}, "Developed technology": {"Fair Value": 50600, "Useful Life": 3}, "Customer relationships": {"Fair Value": 56900, "Useful Life": 2}, "Trade names": {"Fair Value": 19900, "Useful Life": 3}, "Total intangible assets subject to amortization": {"Fair Value": 127400, "Useful Life": ""}}
customer_relationships_fair_value = df["Customer relationships"]["Fair Value"]
developed_technology_fair_value = df["Developed technology"]["Fair Value"]
answer = customer_relationships_fair_value - developed_technology_fair_value