N = int(input())
As = list(map(int, input().split()))
 
sum_humidai = 0
 
for person_idx, person_height in enumerate(As):
  
  if person_idx == 0:
    continue
    
  max_front_person = max(As[:person_idx])
  if max_front_person > person_height:
    humidai = max_front_person - person_height
    sum_humidai += humidai
 
print(sum_humidai)