/in/foo.cc: In function 'int main()':
/in/foo.cc:22:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<=a.size()-1;i++)
              ~^~~~~~~~~~~~
/in/foo.cc:6:11: warning: unused variable 'k' [-Wunused-variable]
     int n,k=0; 
           ^