n=int(input())
l=list(map(int,input().split()))
d=dict()
for i in range(len(l)):
    if (l[i]-i) not in d:
        d[l[i]-i]=1
    elif (l[i]-i) in d:
        d[l[i]-i]+=1
t=0
for j in d.values():
    t+=j*(j-1)//2
print(t)