CF61E Enemy is weak

The Romans have attacked again. This time they are much more than the Persians but Shapur is ready to defeat them. He says: "A lion is never afraid of a hundred sheep". Nevertheless Shapur has to find weaknesses in the Roman army to defeat them. So he gives the army a weakness number. In Shapur's opinion the weakness of an army is equal to the number of triplets $ i,j,k $ such that $ i<j<k $ and $ a_{i}>a_{j}>a_{k} $ where $ a_{x} $ is the power of man standing at position $ x $ . The Roman army has one special trait — powers of all the people in it are distinct. Help Shapur find out how weak the Romans are.

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