Postman and Letters

There is a street of size $N$. The street is denoted by string s that consists of : 'l' (small L) - denotes a letter to be picked by any postman. 'p' - denotes the postman '.' - denotes none of them. In a unit length either a letter is present or a postman or nothing. All letters are to be posted compulsorily so each of them should be picked up by any one postman. In moving from