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riskroll.pl will roll the correct number of dice and score them for a given match up of opponents.

(As stated on Wikipedia)

The attacking player attacks with one, two, three or more armies, rolling a corresponding one, two or three die. At least one army must remain behind in the attacking territory not involved in the attack, as a territory may never be left unoccupied. The defending player must resist the attack with one or two armies (using at most the number of armies currently occupying the defended territory) by rolling a corresponding one or two die. The attacker's highest die number is compared against the defender's highest die. The highest number wins, with the defender winning in the event of a tie. The attacker's next highest die is compared against the defender's second-highest die (assuming the defender committed a second army). Any extra dice (dice not matched against a defending army) are disregarded and do not affect the results. With each dice comparison, the loser removes one army from his territory from the game board.