n = int(input())
nums = list(map(int, input().split()))
d = {}

for i, val in enumerate(nums):
    diff = val - i
    d[diff] = d.get(diff, 0) + 1

count = 0
for k in d.values():       
    count += k * (k-1)//2         

print(count)

这题哪里入门啦！这感觉比很多简单级别的题目都难