n = int(input())
A = list(map(int, input().split()))

ans = 0
for k in range(1, 2001):
    DP = [[0 for _ in range(2001)] for _ in range(5)]
    DP[0][0] = 1
    for i in range(n):
        a = A[i]
        DP[1][a] += DP[0][0]
        if 0 < a - k - 10 <= 2000:
            DP[2][a] += DP[1][a - k - 10]
        if 0 < a + k <= 2000:
            DP[3][a] += DP[2][a + k]
        if 0 < a - k - 1 <= 2000:
            DP[4][a] += DP[3][a - k - 1]
    ans += sum(DP[4])
print(ans)