/in/foo.cc: In function 'int main()':
/in/foo.cc:33:18: error: unable to find numeric literal operator 'operator""n'
   for(int i=1;i<=2n;++i){
                  ^~
/in/foo.cc:33:18: note: use -std=gnu++11 or -fext-numeric-literals to enable more built-in suffixes