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)