/in/foo.cc:6:11: error: '::main' must return 'int'
 void main()
           ^
/in/foo.cc: In function 'int main()':
/in/foo.cc:21:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < B.size(); i++)
                  ~~^~~~~~~~~~
/in/foo.cc:26:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < B.size(); i++)
                  ~~^~~~~~~~~~