prepare (1/3): 
finished

make (2/3): 
g++ -O2 -Wall -o hw281_4.o -c hw281_4.cpp 
g++ -O2 -Wall -o trade.o -c trade.cpp 
g++ -O2 -Wall -o main hw281_4.o trade.o
finished

clean (3/3): 
finished

all task finished, build successfully