import sys, re

from collections import deque, defaultdict, Counter

from math import ceil, sqrt, hypot, factorial, pi, sin, cos, radians, gcd

from itertools import accumulate, permutations, combinations, product, groupby, combinations_with_replacement

from operator import itemgetter, mul

from copy import deepcopy

from string import ascii_lowercase, ascii_uppercase, digits

from bisect import bisect, bisect_left

from heapq import heappush, heappop

from functools import reduce

def input(): return sys.stdin.readline().strip()

def INT(): return int(eval(input()))

def MAP(): return list(map(int, input().split()))

def LIST(): return list(map(int, input().split()))

def ZIP(n): return list(zip(*(MAP() for _ in range(n))))

sys.setrecursionlimit(10 ** 9)

INF = float('inf')

mod = 10 ** 9 + 7



N = INT()

A = LIST()



A = [x-i for i, x in enumerate(A)]

cnt_A = Counter(A)



ans = 0

for i, a in enumerate(A):

    ans += cnt_A[-A[i]-2*i]

print(ans)

import sys, re

from collections import deque, defaultdict, Counter

from math import ceil, sqrt, hypot, factorial, pi, sin, cos, radians, gcd

from itertools import accumulate, permutations, combinations, product, groupby, combinations_with_replacement

from operator import itemgetter, mul

from copy import deepcopy

from string import ascii_lowercase, ascii_uppercase, digits

from bisect import bisect, bisect_left

from heapq import heappush, heappop

from functools import reduce

def input(): return sys.stdin.readline().strip()

def INT(): return int(eval(input()))

def MAP(): return list(map(int, input().split()))

def LIST(): return list(map(int, input().split()))

def ZIP(n): return list(zip(*(MAP() for _ in range(n))))

sys.setrecursionlimit(10 ** 9)

INF = float('inf')

mod = 10 ** 9 + 7



N = INT()

A = LIST()



L = [i+x for i, x in enumerate(A)]

R = [i-x for i, x in enumerate(A)]

cnt_R = Counter(R)



ans = 0

for l in L:

	ans += cnt_R[l]

print(ans)

import sys, re
from collections import deque, defaultdict, Counter
from math import ceil, sqrt, hypot, factorial, pi, sin, cos, radians, gcd
from itertools import (
    accumulate,
    permutations,
    combinations,
    product,
    groupby,
    combinations_with_replacement,
)
from operator import itemgetter, mul
from copy import deepcopy
from string import ascii_lowercase, ascii_uppercase, digits
from bisect import bisect, bisect_left
from heapq import heappush, heappop
from functools import reduce


def input():
    return sys.stdin.readline().strip()


def INT():
    return int(eval(input()))


def MAP():
    return list(map(int, input().split()))


def LIST():
    return list(map(int, input().split()))


def ZIP(n):
    return list(zip(*(MAP() for _ in range(n))))


sys.setrecursionlimit(10**9)
INF = float("inf")
mod = 10**9 + 7
N = INT()
A = LIST()
A = [x - i for i, x in enumerate(A)]
cnt_A = Counter(A)
ans = 0
for i, a in enumerate(A):
    ans += cnt_A[-A[i] - 2 * i]
print(ans)

import sys, re
from collections import deque, defaultdict, Counter
from math import ceil, sqrt, hypot, factorial, pi, sin, cos, radians, gcd
from itertools import (
    accumulate,
    permutations,
    combinations,
    product,
    groupby,
    combinations_with_replacement,
)
from operator import itemgetter, mul
from copy import deepcopy
from string import ascii_lowercase, ascii_uppercase, digits
from bisect import bisect, bisect_left
from heapq import heappush, heappop
from functools import reduce


def input():
    return sys.stdin.readline().strip()


def INT():
    return int(eval(input()))


def MAP():
    return list(map(int, input().split()))


def LIST():
    return list(map(int, input().split()))


def ZIP(n):
    return list(zip(*(MAP() for _ in range(n))))


sys.setrecursionlimit(10**9)
INF = float("inf")
mod = 10**9 + 7
N = INT()
A = LIST()
L = [i + x for i, x in enumerate(A)]
R = [i - x for i, x in enumerate(A)]
cnt_R = Counter(R)
ans = 0
for l in L:
    ans += cnt_R[l]
print(ans)

-A = [x - i for i, x in enumerate(A)] -cnt_A = Counter(A) +L = [i + x for i, x in enumerate(A)] +R = [i - x for i, x in enumerate(A)] +cnt_R = Counter(R) -for i, a in enumerate(A): -    ans += cnt_A[-A[i] - 2 * i] +for l in L: +    ans += cnt_R[l]