from collections import defaultdict

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

ans = 0
for d in range(1, 10**5*2+1):
    D = defaultdict(int)
    e = 10**5*4//d
    for i in range(min(e, N)):
        D[-(A[i]-d*(i+1))] += 1
    for i in range(min(e, N)):
        ans += D[A[i]-d*(i+1)]
        if A[i]-d*(i+1) == -(A[i]-d*(i+1)):
            ans -= 1

print(ans//2)