N = int(input())
A = list(map(int, input().split()))

if A[0]-A[1]>0:
  total = A[0]-A[1]
else:
  total=0 
dif = total
for i in range(1,N):
  if (A[i-1]+dif)-A[i]>0:
    total += (A[i-1]+dif)-A[i]
    dif = (A[i-1]+dif)-A[i]
  else:
    dif = 0
print(total)