import os
import sys
input = lambda:sys.stdin.readline().strip()
minput = lambda:map(int, input().split())
n = int(input())
a = minput()
prev = 1
d = []
for i in a:
    d.append(i - prev)
    prev = i
r = 0
t = [i for i in d if i > 0]
print(sum(t))