n = int(input())
a = list(map(int, input().split()))

for i in range(n):
    a[i] -= i
a.append(12345678987654321)

res = 0

p = -1
l = -1
for i in range(n + 1):
    if a[i] == p:
        None
    else:
        w = i - l
        res += w * (w - 1) // 2
        p = a[i]
        l = i

print(res)