Playing RisiKo!

In a game of RisiKo! (the Italian version of the popular board game Risk!), the players throw six-sided dice to conquer territories around a World map.

When two players contend a territory there is a battle, and they throw from 1 up to 3 dice, with each die being an army sent to fight. To establish who loses armies after the battle, the dice are compared starting from the highest value, and proceeding with the comparisons in descending order (eliminating the dice in excess if the amount of thrown dice among the two players is different).

When the rolls are compared, for each comparison the die of the attacking player must have a greater value than the defender's die to win the fight. In the case of a tie, the defender wins the single fight. The loser only loses 1 army per comparison.

Given two arrays attacker (rolls of the attacker) and defender (rolls of the defender), implement a function that returns the armies lost by the defender as an integer.

Examples

risiko([3, 6, 4], [2, 5, 3]) ➞ 3
# Comparison 1:
# ATT(6) vs. DEF(5): DEF loses an army
# Comparison 2:
# ATT(4) vs. DEF(3): DEF loses an army
# Comparison 3:
# ATT(3) vs. DEF(2): DEF loses an army

risiko([3, 6], [6, 4, 4]) ➞ 0
# Comparison 1:
# ATT(6) vs. DEF(6): ATT loses an army
# Comparison 2:
# ATT(3) vs. DEF(4): ATT loses an army

risiko([3, 1], [1]) ➞ 1
# Comparison 1:
# ATT(3) vs. DEF(1): DEF loses an army

Notes

• When two compared dice have the same value, the player who defends wins the fight (see example #2).
• When the number of rolls is different between the two players, the number of comparisons to do is equal to the lowest amount of rolled dice (see examples #2 and #3).
• You can expect only valid inputs: each array will have at least a die, and no more than three dice, with values in the range from 1 to 6.