View Compilation Error

0_0_16526227_9830.cpp: In function 'int main()':
0_0_16526227_9830.cpp:9:65: error: 'sqrt' was not declared in this scope
   float res = sqrt((x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1));
                                                                 ^