fastest way to get if 2 circles are overlapping

overlap.cs

float dx = c2.Position.x - c1.Position.x;
float dy = c2.Position.y - c1.Position.y;
float radii = r1.Radius + r2.Radius;
if ((dx * dx) + (dy * dy) < radii * radii){
  return true;
}
else{
  return false;
}