foo.cc: In function ‘int main()’:
foo.cc:16:22: error: ‘sqrt’ was not declared in this scope
   16 |                 d += sqrt((x - x1) *(x - x1) + (y - y1) *(y - y1));
      |                      ^~~~
foo.cc:19:13: error: ‘sqrt’ was not declared in this scope
   19 |         d+= sqrt((x - x0) *(x - x0) + (y - y0) *(y - y0));
      |             ^~~~