Buggy python submission (verdict: Runtime Error):
#MLE注意！0や1のケースに注意

def main():
    import sys
    input = sys.stdin.readline
    sys.setrecursionlimit(10**7)
    from collections import Counter, deque
    #from collections import defaultdict
    from itertools import combinations, permutations, accumulate, groupby, product
    from bisect import bisect_left,bisect_right
    from heapq import heapify, heappop, heappush
    import math
    #from math import gcd

    #inf = 10**17
    #mod = 10**9 + 7

    n = int(input())
    a = list(map(int, input().split()))
    res = 0
    l = [0]*(10**6)
    used = []
    for i in range(n):
        if 0<=i+1-a[i]<n:
            l[i+1-a[i]] += 1
            used.append(i+1-a[i])
    cnt = 0
    for i in range(n):
        if 0<=i+1-a[i]<n:
            l[used[cnt]] -= 1
            cnt += 1
        b = i+1+a[i]
        res += l[b]
    print(res)



if __name__ == '__main__':
    main()

Fixed submission:
#MLE注意！0や1のケースに注意

def main():
    import sys
    input = sys.stdin.readline
    sys.setrecursionlimit(10**7)
    from collections import Counter, deque
    #from collections import defaultdict
    from itertools import combinations, permutations, accumulate, groupby, product
    from bisect import bisect_left,bisect_right
    from heapq import heapify, heappop, heappush
    import math
    #from math import gcd

    #inf = 10**17
    #mod = 10**9 + 7

    n = int(input())
    a = list(map(int, input().split()))
    res = 0
    l = [0]*(10**6)
    used = []
    for i in range(n):
        if 0<=i+1-a[i]<n:
            l[i+1-a[i]] += 1
            used.append(i+1-a[i])
    cnt = 0
    for i in range(n):
        if 0<=i+1-a[i]<n:
            l[used[cnt]] -= 1
            cnt += 1
        if 0<=i+1+a[i]<10**6:
            b = i+1+a[i]
            res += l[b]
    print(res)



if __name__ == '__main__':
    main()

Buggy python submission (verdict: Runtime Error):
#MLE注意！0や1のケースに注意

def main():
    import sys
    input = sys.stdin.readline
    sys.setrecursionlimit(10**7)
    from collections import Counter, deque
    #from collections import defaultdict
    from itertools import combinations, permutations, accumulate, groupby, product
    from bisect import bisect_left,bisect_right
    from heapq import heapify, heappop, heappush
    import math
    #from math import gcd

    #inf = 10**17
    #mod = 10**9 + 7

    n = int(input())
    a = list(map(int, input().split()))
    res = 0
    l = [0]*(10**6)
    used = []
    for i in range(n):
        if a[i] >= n:
            continue
        l[i+1-a[i]] += 1
        used.append(i+1-a[i])
    for i in range(n):
        if a[i] >= n:
            continue
        l[used[i]] -= 1
        b = i+1+a[i]
        res += l[b]
    print(res)



if __name__ == '__main__':
    main()

Fixed submission:
#MLE注意！0や1のケースに注意

def main():
    import sys
    input = sys.stdin.readline
    sys.setrecursionlimit(10**7)
    from collections import Counter, deque
    #from collections import defaultdict
    from itertools import combinations, permutations, accumulate, groupby, product
    from bisect import bisect_left,bisect_right
    from heapq import heapify, heappop, heappush
    import math
    #from math import gcd

    #inf = 10**17
    #mod = 10**9 + 7

    n = int(input())
    a = list(map(int, input().split()))
    res = 0
    l = [0]*(10**6)
    used = []
    for i in range(n):
        if 0<=i+1-a[i]<n:
            l[i+1-a[i]] += 1
            used.append(i+1-a[i])
    cnt = 0
    for i in range(n):
        if 0<=i+1-a[i]<n:
            l[used[cnt]] -= 1
            cnt += 1
        if 0<=i+1+a[i]<10**6:
            b = i+1+a[i]
            res += l[b]
    print(res)



if __name__ == '__main__':
    main()

Buggy python submission (verdict: Runtime Error):
#MLE注意！0や1のケースに注意

def main():
    import sys
    input = sys.stdin.readline
    sys.setrecursionlimit(10**7)
    from collections import Counter, deque
    #from collections import defaultdict
    from itertools import combinations, permutations, accumulate, groupby, product
    from bisect import bisect_left,bisect_right
    from heapq import heapify, heappop, heappush
    import math
    #from math import gcd

    #inf = 10**17
    #mod = 10**9 + 7

    n = int(input())
    a = list(map(int, input().split()))
    res = 0
    l = [0]*(10**6)
    used = []
    for i in range(n):
        l[i+1-a[i]] += 1
        used.append(i+1-a[i])
    for i in range(n):
        l[used[i]] -= 1
        b = i+1+a[i]
        res += l[b]
    print(res)



if __name__ == '__main__':
    main()

Fixed submission:
#MLE注意！0や1のケースに注意

def main():
    import sys
    input = sys.stdin.readline
    sys.setrecursionlimit(10**7)
    from collections import Counter, deque
    #from collections import defaultdict
    from itertools import combinations, permutations, accumulate, groupby, product
    from bisect import bisect_left,bisect_right
    from heapq import heapify, heappop, heappush
    import math
    #from math import gcd

    #inf = 10**17
    #mod = 10**9 + 7

    n = int(input())
    a = list(map(int, input().split()))
    res = 0
    l = [0]*(10**6)
    used = []
    for i in range(n):
        if 0<=i+1-a[i]<n:
            l[i+1-a[i]] += 1
            used.append(i+1-a[i])
    cnt = 0
    for i in range(n):
        if 0<=i+1-a[i]<n:
            l[used[cnt]] -= 1
            cnt += 1
        if 0<=i+1+a[i]<10**6:
            b = i+1+a[i]
            res += l[b]
    print(res)



if __name__ == '__main__':
    main()