# import pypyjit;pypyjit.set_param("max_unroll_recursion=-1")
# from bisect import *
from collections import *

# from heapq import *
# from itertools import *
# from math import *
# from datetime import *
# from decimal import*
# from string import ascii_lowercase,ascii_uppercase
# import numpy as np
import sys
import os

# sys.setrecursionlimit(10**6)
INF = 10**18
MOD = 998244353
# MOD = 10**9 + 7
File = open("input.txt", "r") if os.path.exists("input.txt") else sys.stdin


def input():
    return File.readline()[:-1]


# ///////////////////////////////////////////////////////////////////////////


N = int(input())
A = list(map(int, input().split()))
l = deque([[0, 0]])
for i in range(N):
    if i == 0:
        deque.append(l, [A[i], 1])
        continue
    if A[i] - A[i - 1] == 1:
        l[-1][1] += 1
    else:
        deque.append(l, [A[i], 1])

ans = 0
for i in range(len(l) - 1):
    pop = deque.pop(l)
    ans += pop[0] - l[-1][0] - 1


print(ans)