/in/foo.cc: In function 'int main()':
/in/foo.cc:54:16: error: 'sort' was not declared in this scope
     sort(a, a+M);
                ^
/in/foo.cc: In function 'int bfs()':
/in/foo.cc:44:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^