سوال آب - کوئرا

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

mx2 = 0 # max number before item
mx = max(a) # max number in array

for i in range(n):
    if a[i] > mx2:
        mx2 = a[i] # update max number before item
    if i > a.index(mx):
        mx = max(a[i:]) # max number after item
    r = min(mx, mx2) - a[i]

    result += (r) if r > 0 else 0

print(result)