N = map(int, input().split())
l = list(map(int, input().split()))
from collections import deque


l = deque(l)

count=0
for i in range(len(l)-1):
    if l[i] >l[i+1]:
        while l[i] >l[i+1]:
            l[i+1]+=1
            count+=1

print(count)