import sys

n = int(sys.stdin.readline().strip())
a = list(map(int,sys.stdin.readline().split()))

d = {}

for i in range(n):
    x = a[i] -i
    d[x] = d.get(x,0) + 1

m = 0
for i in d.values():
    m += i*(i-1)//2
print(m)