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": 34600, "Useful Life": 5}, "Customer relationships": {"Fair Value": 16900, "Useful Life": 7}, "Backlog": {"Fair Value": 1400, "Useful Life": 3}, "Total intangible assets subject to amortization": {"Fair Value": 52900, "Useful Life": ""}}
developed_technology_fair_value = df["Developed technology"]["Fair Value"]
customer_relationships_fair_value = df["Customer relationships"]["Fair Value"]
answer = developed_technology_fair_value - customer_relationships_fair_value