foo.cc: In function 'int main()':
foo.cc:7:13: error: 'n' cannot be used as a function
  cout<<n(n+1)(2n+1)/6<<endl;
             ^
foo.cc:7:15: error: unable to find numeric literal operator 'operator""n'
  cout<<n(n+1)(2n+1)/6<<endl;
               ^~
foo.cc:7:15: note: use -std=gnu++11 or -fext-numeric-literals to enable more built-in suffixes