Amazing
-------
Maria is preparing for the American Competition of Mazes (ACM).
Contestants have to solve mazes such as

******1***
*23141231*
*2******7*
*12312311*
******2***

Each maze has a single entrance at the top and a single exit at 
the bottom.  A maze may have multiple paths from the entrance 
to the exit.  Contestants have to find a path with the smallest 
cost (the cost of a path is the sum of the integers that make 
up the path).  For the above example, the smallest cost is
1 + 2 + 3 + 1 + 7 + 1 + 1 + 3 + 2 = 21.

As Maria is preparing for the competition, she is solving more
and more complex mazes.  Her dad has great difficulty checking
if her solutions are correct.  Can you help by writing a program
that takes a maze as input and produces the smallest cost?

Input
-----
The input may contain multiple test cases.

The first line of each test case contains two integers R (3 <= R <= 1000),
and C (3 <= C <= 1000) where R indicates the number of rows of the maze
and C represents the number of columns of the maze.

The next R lines each contain a row of the maze.  Each row consists of
C characters.  The first and last character are always a *.  The remaining
characters can either be a digit or a *.  The first and the last row only
contains a single character different from *.

The input will terminate with two zeros for R and C.

Output
------
For each maze in the input, print the smallest cost.  If no path from
the entrance to the exit exists, print impossible.

Sample Input
------------
5 10
******1***
*23141231*
*2******7*
*12312311*
******2***
3 3
*1*
***
*8*
0 0

Sample Output
-------------
21
impossible