A milk vendor generally sells 3 Grades of milk. Grade I is pure milk with no water mixed in it, Grade II is a mixture of milk and water in the ratio 3 ∶ 2 and Grade III is a mixture of milk and water in the ratio 2 ∶ 3. On a particular day he has x liters of Grade I and 3 liters of Grade III milk and he got an order to supply 7 liters of Grade II milk. The minimum value of x (in litres) required to prepare 7 lts of Grade II milk by mixing Grade I milk, Grade III milk and water, is