View Compilation Error

0_0_38550756_15440.cpp: In function 'int main(int, char**)':
0_0_38550756_15440.cpp:8:40: error: 'sqrt' was not declared in this scope
  s=sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
                                        ^