Negative Dice

If the odd numbers on two dice are made negative, which of the totals cannot be achieved?
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Problem



I have some strange dice: the faces show the numbers 1 to 6 as usual, except that the odd numbers are negative (i.e. -1, -3, -5 in place of 1, 3, 5).

If I throw two such dice, which of the following totals cannot be achieved?

a) 3

b) 7

c) 8

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