shu = int(input())
l = list(map(int, input().split()))
s = 0
dic = {}
for i in range(len(l)):
    a = l[i] - i - 1
    dic[a] = dic.get(a,0) + 1
for n in dic.values():
    if n >= 2:
        s += n*(n-1)//2
print(s)