View Compilation Error

0_0_31936738_23123.cpp: In function 'int main()':
0_0_31936738_23123.cpp:8:53: error: 'sqrt' was not declared in this scope
   l=sqrt((x1*x1-2*x1*x2+x2*x2)-(y1*y1-2*y1*y2+y2*y2));
                                                     ^