foo.cc: In function ‘double ojld(int, int)’:
foo.cc:18:12: error: ‘sqrt’ was not declared in this scope
   18 |     return sqrt((p[i].x-p[j].x)*(p[i].x-p[j].x)+(p[i].y-p[j].y)*(p[i].y-p[j].y));
      |            ^~~~