CF131B Opposites Attract

Everybody knows that opposites attract. That is the key principle of the "Perfect Matching" dating agency. The "Perfect Matching" matchmakers have classified each registered customer by his interests and assigned to the $ i $ -th client number $ t_{i} $ ( $ -10

The first line of the input data contains an integer $ n $ ( $ 1

Print the number of couples of customs with opposite $ t $ . The opposite number for $ x $ is number $ -x $ ( $ 0 $ is opposite to itself). Couples that only differ in the clients' order are considered the same. Note that the answer to the problem can be large enough, so you must use the 64-bit integer type for calculations. Please, do not use the %lld specificator to read or write 64-bit integers in С++. It is preferred to use cin, cout streams or the %I64d specificator.

In the first sample the couples of opposite clients are: (1,2), (1,5) и (3,4). In the second sample any couple of clients is opposite.