import sys
from collections import Counter

def main():
    input = sys.stdin.read().split()
    n = int(input[0])
    a = list(map(int, input[1:n+1]))
    counter = Counter(a)
    s = n + 1
    ans = 0
    for x in counter:
        target = s - x
        if x < target:
            ans += counter[x] * counter.get(target, 0)
        elif x == target:
            ans += counter[x] * (counter[x] - 1) // 2
    print(ans)

if __name__ == "__main__":
    main()