Problem C - Money Money
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Shopkeeper: 100 dollars and 80 cents, please.
Customer: Here you are. (handing a 200-dollar bill)
Shopkeeper: Would you have 80 cents? (passing back a 100-dollar bill)
Customer: No, but here you are. (adding a 1-dollar coin)
Shopkeeper: Sorry, but I have only this. (showing a shiny 50-cent coin)
Customer: So I need to give you another 30 cents. But I have only 40.
Shopkeeper: That's ok, here are the remaining 10.

Have you ever experienced similar situations? Paying a precise amount
can sometimes be difficult, if the set of available coins and bills
("tenders") is limited. The situation above was finally solved: The
customer paid 200+1 dollars, got 100+0.50 back, paid another 0.20+0.20,
and finally got 0.10 back. This means, 7 tenders had to be exchanged.
Sometimes, it may be even more complicated. Your task is to write a
program that solves situations like this.

Input
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Input contains several tasks to be solved. Each task begins with a line
containing one non-negative number: the amount to be paid. Then there is
a list of tenders that the customer has. Each line in the list contains
the tender nominal value (non-negative number), one space, number of
tenders of that value (non-negative integer), and the lowercase letter
"x". The list is terminated by a line containing the number -1.

After the first list, there is a list of tenders that the shopkeeper
has.  The second list has the exactly same format as the first one.

Then the next task begins. The last task is followed by one more line
containing -1.

You may assume that each list will contain at most 100 lines, and nobody
will have more than 10000 units and/or 500 tenders. Nominal values can
be arbitrary, they do not need to follow any existing scheme valid in
known countries. All numbers that allow non-integer values will be given
either as integers or as decimal numbers with one or two digits after
the decimal point.

Output
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For each task, output one line containing the sentence "X tenders must
be exchanged.", with X replaced by the minimal number of tenders that
are to be used to pay the required amount. If this is not possible at
all, output the sentence "The payment is impossible." instead.

Sample Input
------------

100.80
500 1x
200 3x
1.00 10x
0.20 2x
-1
500 10x
200 12x
100 8x
0.10 1x
0.20 0x
0.50 100x
20 2x
-1
200
10 19x
-1
200 1x
-1
-1

Output for Sample Input
-----------------------

7 tenders must be exchanged.
The payment is impossible.