n = int(input())
a = [int(i) for i in input().split(' ')]
result = 0
max_height = 0

for i in range(n):
  an = a[i]
  if i == 0:
    max_height = an
    max_height_idx = 0
    continue

  if an >= max_height:
    max_height = an
    max_height_idx = i
    continue

  max_height = max(a[max_height_idx:i])
  result += (max_height - an)

print(result)