View Compilation Error

0_0_17021437_32445.cpp: In function 'double dis(Point, Point)':
0_0_17021437_32445.cpp:6:64: error: 'sqrt' was not declared in this scope
     return sqrt((p1.x-p2.x)*(p1.x-p2.x)+(p1.y-p2.y)*(p1.y-p2.y));
                                                                ^