from collections import defaultdict

n = int(input())
a = list(map(int, input().split(' ')))

ans = 0

for d in range(1, 2000):
    mp = defaultdict(int)
    for i in range(n):
        mp[(a[i], 0)] += 1
        if (a[i] - 10 - d, 0) in mp:
            mp[(a[i], 1)] += mp[(a[i] - 10 - d, 0)]
        if (a[i] + d, 1) in mp:
            mp[(a[i], 2)] += mp[(a[i] + d, 1)]
        if (a[i] - d - 1, 2) in mp:
            ans += mp[(a[i] - d - 1, 2)]
print(ans)