prepare (1/3): 
finished

make (2/3): 
g++ -g -Wall -O2 -std=c++11 -o main.o -c main.cpp
main.cpp: In function 'int main(int, char**)':
main.cpp:16:12: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
     freopen("g1.in", "r", stdin);
     ~~~~~~~^~~~~~~~~~~~~~~~~~~~~
g++ -g -Wall -O2 -std=c++11 -o graph.o -c graph.cpp
g++ -g -Wall -O2 -std=c++11 -o main main.o graph.o
finished

clean (3/3): 
finished

all task finished, build successfully