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*<sub class="lower-index">*i*</sub> ( - 10 ≤ *t*<sub class="lower-index">*i*</sub> ≤ 10). Of course, one number can be assigned to any number of customers.

"Perfect Matching" wants to advertise its services and publish the number of opposite couples, that is, the couples who have opposite values of *t*. Each couple consists of exactly two clients. The customer can be included in a couple an arbitrary number of times. Help the agency and write the program that will find the sought number by the given sequence *t*<sub class="lower-index">1</sub>, *t*<sub class="lower-index">2</sub>, ..., *t*<sub class="lower-index">*n*</sub>. For example, if *t* = (1,  - 1, 1,  - 1), then any two elements *t*<sub class="lower-index">*i*</sub> and *t*<sub class="lower-index">*j*</sub> form a couple if *i* and *j* have different parity. Consequently, in this case the sought number equals 4.

Of course, a client can't form a couple with him/herself.
The first line of the input data contains an integer *n* (1 ≤ *n* ≤ 10<sup class="upper-index">5</sup>) which represents the number of registered clients of the "Couple Matching". The second line contains a sequence of integers *t*<sub class="lower-index">1</sub>, *t*<sub class="lower-index">2</sub>, ..., *t*<sub class="lower-index">*n*</sub> ( - 10 ≤ *t*<sub class="lower-index">*i*</sub> ≤ 10), *t*<sub class="lower-index">*i*</sub> — is the parameter of the *i*-th customer that has been assigned to the customer by the result of the analysis of his interests.
5
-3 3 0 0 3
3
3
0 0 0
3
1
0
0
1
5
0
2
0 0
1
2
-3 3
1
2
1 5
0
4
1 -1 1 -1
4
10
1 -1 2 -2 3 -3 4 -4 0 0
5
5
0 0 0 0 0
10
4
4 4 4 -1
0
2
10 -10
1
2
-10 10
1
3
-1 -1 2
0
2
-1 2
0
7
0 10 -10 10 -10 10 10
8
5
-3 2 1 -1 3
2
4
-10 10 -10 10
4
4
1 -1 2 -2
2
n = int(input())
nums = input()

pos = []
neg = []
zeroes = 0

for num in nums.split(" "):
    if int(num) > 0:
        pos.append(int(num))
    elif int(num) < 0:
        neg.append(int(num))
    else:
        zeroes += 1

# Since 0 can be matches with any other zero, this can be achieved with the combinations formula
matches = zeroes * (zeroes - 1) // 2

# Now for postitive and negative numbers
for a in range(len(pos)):
    for b in range(len(neg)):
        if -pos[a] == neg[b]:
            matches += 1
print(matches)
