/in/foo.cc: In function 'bool operator<(P, P)':
/in/foo.cc:23:49: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
 bool operator<(P a,P b){return a.x<b.x||a.x==b.x&&a.y<b.y;}
                                         ~~~~~~~~^~~~~~~~~
/in/foo.cc: In function 'bool cross()':
/in/foo.cc:69:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<v.size();++i){
              ~^~~~~~~~~