Based on fair value, what is the difference in value between U.S. treasury securities and agency securities?
{"Debt Investments -- U.S. treasury securities": {"Amortized Cost": 52809, "Unrealized Gains": "$\u2014", "Unrealized Losses": -109, "Fair Value": 52700}, "Debt Investments -- Agency securities": {"Amortized Cost": 22097, "Unrealized Gains": "\u2014", "Unrealized Losses": -82, "Fair Value": 22015}, "Debt Investments -- Certificates of deposit and time deposits": {"Amortized Cost": 58824, "Unrealized Gains": "\u2014", "Unrealized Losses": "\u2014", "Fair Value": 58824}, "Debt Investments -- Commercial paper": {"Amortized Cost": 35372, "Unrealized Gains": "\u2014", "Unrealized Losses": "\u2014", "Fair Value": 35372}, "Debt Investments -- Corporate debt securities": {"Amortized Cost": 158538, "Unrealized Gains": 14, "Unrealized Losses": -669, "Fair Value": 157883}, "Debt Investments -- Total investments": {"Amortized Cost": 327640, "Unrealized Gains": 14, "Unrealized Losses": -860, "Fair Value": 326794}}
us_treasury_securities_fair_value = df["Debt Investments -- U.S. treasury securities"]["Fair Value"]
agency_securities_fair_value = df["Debt Investments -- Agency securities"]["Fair Value"]
answer = us_treasury_securities_fair_value - agency_securities_fair_value