View Compilation Error

0_0_21935322_28542.cpp: In function 'double dis(Point, Point)':
0_0_21935322_28542.cpp:11:67: error: 'sqrt' was not declared in this scope
  return sqrt((a.x - b.x) * (a.x - b.x) + (a.y - b.y) * (a.y - b.y));
                                                                   ^