16. An airline company operates a fleet of airplanes. If it buys an airplane, it can be run up to three years. For safety reasons, the plane should be replaced at the end of the third year. The airline evaluates the heath of the airplanes every year, and if an airplane fails the test, it is replaced in that year itself. The replacement costs in million euros are depicted in the network diagram below. Nodes represent the beginning of a year.

60 65 40 25 30 15 2 20 3 4 5 65 40