What is the difference in useful life between developed technology and customer relationships?
{"": {"Fair Value": "(U.S. $ in thousands)", "Useful Life": "(years)"}, "Developed technology": {"Fair Value": 35600, "Useful Life": 5}, "Customer relationships": {"Fair Value": 48600, "Useful Life": 10}, "Trade name": {"Fair Value": 3700, "Useful Life": 5}, "Total intangible assets subject to amortization": {"Fair Value": 87900, "Useful Life": ""}}
customer_relationships_useful_life = df["Customer relationships"]["Useful Life"]
developed_technology_useful_life = df["Developed technology"]["Useful Life"]
answer = customer_relationships_useful_life - developed_technology_useful_life