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)
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)