import sys
import math
data = sys.stdin.read().split()
n = int(data[0])
nums = [int(x) for x in data[1:]]
dic = {}
for i in range(n):
key = nums[i]-i
dic[key] = dic.get(key, 0)+1
res = 0
for key in dic:
if dic[key] > 1:
res += math.comb(dic[key], 2)
print(res)
from collections import Counter
n = int(input())
numbers = list(map(int, input().split()))
b = [numbers[i] - (i+1) for i in range(n)]
counter = Counter(b)
count = 0
for freq in counter.values():
if freq >1:
count += freq*(freq - 1) //2
print(count)