/in/foo.cc: In function 'll back1(int, ll)':
/in/foo.cc:34:2: error: return-statement with no value, in function returning 'll {aka long long int}' [-fpermissive]
  return ;
  ^~~~~~
/in/foo.cc: In function 'll back(int, ll)':
/in/foo.cc:47:2: error: return-statement with no value, in function returning 'll {aka long long int}' [-fpermissive]
  return ;
  ^~~~~~