View Compilation Error

0_0_21756724_27898.cpp: In function 'double angle(Point)':
0_0_21756724_27898.cpp:25:51: error: 'pi' was not declared in this scope
  if (a.x>0 && a.y>0)   return atan(y / x)*180.0 / pi;
                                                   ^
0_0_21756724_27898.cpp:26:63: error: 'pi' was not declared in this scope
  else if (a.x<0 && a.y>0)  return 180.0 - atan(y / x)*180.0 / pi;
                                                               ^
0_0_21756724_27898.cpp:27:63: error: 'pi' was not declared in this scope
  else if (a.x<0 && a.y<0)  return 180.0 + atan(y / x)*180.0 / pi;
                                                               ^
0_0_21756724_27898.cpp:28:63: error: 'pi' was not declared in this scope
  else if (a.x>0 && a.y<0)  return 360.0 - atan(y / x)*180.0 / pi;
                                                               ^