## https://yukicoder.me/problems/no/3257


    

def main():
    N = int(input())
    A = list(map(int, input().split()))

    max_a = max(A)

    dp = [{} for _ in range(2 * max_a + 1)]
    answer = 0
    for i in range(N):
        a = A[i]
        i0 = i + 1

        # answer計算
        b = -a + i0
        m = 1
        while max_a >= b:
            if m in dp[b + max_a]:
                answer += dp[b + max_a][m]
            b += i0
            m += 1

        # dp更新
        b = a - i0
        m = 1
        while -max_a <= b:
            if m not in dp[b + max_a]:
                dp[b + max_a][m] = 0
            dp[b + max_a][m] += 1
            b -= i0
            m += 1

    print(answer)


if __name__ == "__main__":
    main()