View Compilation Error

0_0_32293963_15866.cpp: In function 'int main()':
0_0_32293963_15866.cpp:8:63: error: 'sqrt' was not declared in this scope
  double a=sqrt(double((y2-y1)*(y2-y1))+double((x2-x1)*(x2-x1)));
                                                               ^