View Compilation Error

0_0_25389770_1626.cpp: In function 'int main()':
0_0_25389770_1626.cpp:9:53: error: 'sqrt' was not declared in this scope
   m = sqrt((x2 - x1)*(x2 - x1) + (y2 - y1)*(y2 - y1));
                                                     ^