import bisect
import copy
import decimal
import fractions
import heapq
import itertools
import math
import random
import sys
import time
from collections import Counter,deque,defaultdict
from functools import lru_cache,reduce
from heapq import heappush,heappop,heapify,heappushpop,_heappop_max,_heapify_max
def _heappush_max(heap,item):
    heap.append(item)
    heapq._siftdown_max(heap, 0, len(heap)-1)
def _heappushpop_max(heap, item):
    if heap and item < heap[0]:
        item, heap[0] = heap[0], item
        heapq._siftup_max(heap, 0)
    return item
from math import gcd as GCD
read=sys.stdin.read
readline=sys.stdin.readline
readlines=sys.stdin.readlines
write=sys.stdout.write

def Run_Length(lst):
    run_length=[]
    if lst:
        prev=lst[0]
        cnt=1
        for x in lst[1:]:
            if x==prev:
                cnt+=1
            else:
                run_length.append((prev,cnt))
                prev=x
                cnt=1
        run_length.append((prev,cnt))
    return run_length

N=int(readline())
A=list(map(int,readline().split()))
for i in range(N):
    A[i]-=i
ans=0
for a,c in Run_Length(A):
    ans+=c*(c+1)//2
ans-=N
print(ans)