E - This Message Will Self-Destruct in 5s https://atcoder.jp/contests/abc166/tasks/abc166_e

abc166_e.py

N = int(input())
A = list(map(int, input().split()))

# j - i = Ai + Aj
# j - Aj = i + Ai
cnt = {}
for j in range(N):
    val = j - A[j]
    if val not in cnt:
        cnt[val] = 0
    cnt[val] += 1

ans = 0
for i in range(N):
    val = i + A[i]
    if val in cnt:
        ans += cnt[val]
        
print(ans)