Why can the fed control the real interest rate in the short run but not in the long run?

S. you are changing planes in london for a flight to paris where you will connect with your flight to capetown. you are picking up reading material for the flight and are looking at the prices listed on the economist magazine which conveniently lists prices in several different global currencies. you note that the price in pounds is 2.40 pounds and the price in euros is 2 euros. the exchange rate for the dollar (your credit card was issued in the usa) is $1.59/pound and s1.3837/euro. should you buy reading materials now or wait until you're in paris?