import sys
from functools import partial
# import pypyjit
# pypyjit.set_param('max_unroll_recursion = -1')

sys.setrecursionlimit(500005)
stdin = sys.stdin

ns = lambda: stdin.readline().strip()
ni = lambda: int(ns())
na = lambda: list(map(int, stdin.readline().split()))
nz = lambda: list(map(lambda x: int(x)-1, stdin.readline().split())) # 遅い
printa = partial(print, sep="\n") # printa(*A) で各要素を改行して出力
mod = 1000000007 # 998244353
inf = 10 ** 18

N = ni()
A = na()

A.insert(0, 0)

ans = 0
for i in range(1, N+1)[::-1]:
    ans += A[i] - A[i-1] - 1

print(ans)