View Compilation Error

0_0_34656863_17545.cpp:6:35: error: 'sqrt' was not declared in this scope
 const double sTwo = 1.0 / sqrt(2.0); //斜向前进一步，在X轴和Y轴上移动的距离
                                   ^
0_0_34656863_17545.cpp: In function 'int main()':
0_0_34656863_17545.cpp:83:36: error: 'sqrt' was not declared in this scope
     double dis = sqrt(x * x + y * y); //距离
                                    ^