/in/foo.cc: In function 'int main()':
/in/foo.cc:17:10: warning: statement has no effect [-Wunused-value]
         u;
          ^
/in/foo.cc:18:10: warning: statement has no effect [-Wunused-value]
         v;
          ^
/in/foo.cc:31:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int i = 0; i < G[u].size(); i++)
                         ~~^~~~~~~~~~~~~