import sys
sys.setrecursionlimit(5*10**5)
input = sys.stdin.readline
from collections import defaultdict, deque, Counter
from heapq import heappop, heappush
from bisect import bisect_left, bisect_right
from math import gcd

n = int(input())
a = list(map(int,input().split()))

ans = 0
for i in range(n-1):
    ans += a[i+1] - a[i] - 1

ans += a[0]-1
print(ans)