View Compilation Error

0_0_35411791_6913.cpp: In function 'int main()':
0_0_35411791_6913.cpp:6:63: error: 'sqrt' was not declared in this scope
         s = sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2));
                                                               ^