Which do you mean Caen or Cannes? Caen is 312 kilometres from Boulogne-sur-Mar and Cannes is 1160 kilometres in another direction.
If you mean Caen, you need to take a train journey as there are NO coach services between the two towns.The train journey involves either a change of train at Amiens and Rouen or take a train to Paris Nord, struggle across Paris to Paris St. Lazaire and take a second train to Caen. Either way it is going to take you around 6 hours.