n = input()
n =int (n)
if n== 0:
    print(0)
else:
    num_group = list(map(int, input().split()))
    counts = {}
    for i in range(n):
        diff = num_group[i]-i
        counts[diff] = counts.get(diff,0)+1
    total_pairs = 0
    for count in counts.values():
        if count > 1:
                     total_pairs += count*(count-1) // 2
    print(total_pairs)