n=int(input())
a=list(map(int,input().split()))
L=max(a)
p=[[] for i in range(L+1)]
for i in range(n):
  for j in range(1,L+1):
    if a[i]-j*(i+1)>=-L:
      p[j]+=[a[i]-j*(i+1)]
    else:
      break
a=0
for i in range(1,L+1):
  d={}
  for v in p[i]:
    if -v in d:
      a+=d[-v]
    if v not in d:
      d[v]=0
    d[v]+=1
print(a)