Supermarket

Time Limit: 2 Seconds
Memory Limit: 65536 KB

Mr. Jones is an exemplary husband. Every Saturday morning Mrs. Jones gives
him a list of goods to be bought from the supermarket and he buys exactly what
he has been asked for, always choosing the brands with lowest prices. But Mr.
Jones hates going to the supermarket on a Saturday, since its aisles are packed
with shoppers. He wants to change the way he does his shopping. Instead of going
to and fro to buy the products on his wife's list, he will try to get the goods
on the list going through each aisle only once, picking up the products in the
exact order given in the list. So he asked you to write a program to help him
with his new style of shopping.

Given the information about products available in the supermarket together with
their prices in the order in which they appear in Mr. Jones' way and the list
of products given by his wife, your program must determine the least cost that
he would pay.

Mr. Jones buys the products in the order in which they appear in Mrs. Jones'
list and he never goes back as he walks down the aisles. Therefore, if he buys
the i-th product on his way as the j-th item on the list, the next product to
be bought is the (j+1)-th item of the list - and it must be bought from the
products that come after i in his path. The figure below shows an example where
products are identified by integers. Note that different brands of the same
product may appear separately. In the example Mr. Jones must buy products 1,1,2,20
(notice that product 1 appears twice in the list). For the example, the least
cost for Mr. Jones following his constraints is 21.30. Notice that with this
new way of shopping it may be impossible for Mr. Jones to buy all the goods
on Mrs. Jones list; in that case, your program should warn Mr. Jones.

**Input**

Your program should process data for several shopping sessions. The first line
in the description of a shopping session contains two integers M and N; M indicates
the number of items in Mrs. Jones' list (1 <= M <= 100) and N represents
the total number of products available in the supermarket (1 <= N <= 100,000).
The next line contains M integers Xi representing the list of products in Mrs.
Jones' list (1 <= Xi <= 100,000, 1 <= i <= M). Then N lines follow,
representing the supermarket products in the order in which they appear in Mr.
Jones' way. Each of those lines contains an integer K and a real P which represent
respectively a product identifier and its price (1 <= K <= 100,000). The
end of input is indicated by M = N = 0.

**Output**

For each shopping session in the input, your program should produce one line
of output, containing the least cost that Mr. Jones would pay. If it is not
possible to buy all the goods for the session, print the word 'Impossible'.
The cost must be printed as a real number with two-digit precision, and the
last decimal digit must be rounded. The input will not contain test cases where
differences in rounding are significant.

**Sample Input**

4 8

1 1 2 20

2 0.29

1 0.30

20 0.15

1 1.00

5 0.05

2 10.00

20 20.00

20 10.00

2 5

1 2

3 1.00

4 1.00

2 0.01

1 1.00

2 1.50

2 3

1 2

2 0.05

1 10.00

1 3.00

0 0

**Sample Output**

21.30

2.50

Impossible

Source:

**South America 2002**
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