Problem:
N persons are standing in a row. The height of the i-th person from the front is A_i.  We want to have each person stand on a stool of some heights - at least zero - so that the following condition is satisfied for every person:  Condition: Nobody in front of the person is taller than the person. Here, the height of a person includes the stool.  Find the minimum total height of the stools needed to meet this goal.

Incorrect submission (verdict: Wrong Answer):
##a = int(stdin.readline().rstrip())
##b, c = [int(x) for x in stdin.readline().rstrip().split()]
##s = stdin.readline().rstrip()

##a = list(map(int,[int(x) for x in stdin.readline().rstrip().split()])) a[0, 1, 2, ...]
##a = [[0] * 2 for _ in range(n)]  a[0,0]






# -*- coding: utf-8 -*-
from sys import stdin
from operator import itemgetter
from collections import deque, Counter
import math
import pprint
from functools import reduce
import numpy as np
import random
import bisect
import copy



MOD = int(1e9+7)
INF = float('inf')
alpha = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]

def keta(kazu): 
    kazu_str = str(kazu)
    kazu_list = [int(kazu_str[i]) for i in range(0, len(kazu_str))]
    return kazu_list

def gcd(*numbers): 
    return reduce(math.gcd, numbers)

def combination(m,n): # mCn
    
    if n > m:
        return 'すまん'
    return math.factorial(m) // (math.factorial(m-n) * math.factorial(n))

def pow_k(x,n):  
    if n == 0:
        return 1
    K = 1
    while n > 1:
        if n % 2 != 0:
            K *= x
        x *= x
        n //= 2
    return K * x

def fact(n): 
    arr = {}
    temp = n
    for i in range(2,int(n**0.5)+1):
        if temp % i == 0:
            cnt = 0
            while temp % i == 0:
                cnt += 1
                temp //= i
            arr[i] = cnt
    if temp != 1:
        arr[temp] = 1
    if arr == {}:
        arr[n] = 1
    return arr
    
def main():
    N = int(stdin.readline().rstrip())
    A = list(map(int,[int(x) for x in stdin.readline().rstrip().split()]))
    A.append(INF)
    ans = 0
    print(A)
    
    for i in range(1,N):
        if A[i] < A[i-1]:
            ans += abs(A[i-1]-A[i])
            A[i] = A[i-1]
        
    print(ans)
            
main()

Fixed, accepted submission:
##a = int(stdin.readline().rstrip())
##b, c = [int(x) for x in stdin.readline().rstrip().split()]
##s = stdin.readline().rstrip()

##a = list(map(int,[int(x) for x in stdin.readline().rstrip().split()])) a[0, 1, 2, ...]
##a = [[0] * 2 for _ in range(n)]  a[0,0]






# -*- coding: utf-8 -*-
from sys import stdin
from operator import itemgetter
from collections import deque, Counter
import math
import pprint
from functools import reduce
import numpy as np
import random
import bisect
import copy



MOD = int(1e9+7)
INF = float('inf')
alpha = ["a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"]

def keta(kazu): 
    kazu_str = str(kazu)
    kazu_list = [int(kazu_str[i]) for i in range(0, len(kazu_str))]
    return kazu_list

def gcd(*numbers): 
    return reduce(math.gcd, numbers)

def combination(m,n): # mCn
    
    if n > m:
        return 'すまん'
    return math.factorial(m) // (math.factorial(m-n) * math.factorial(n))

def pow_k(x,n):  
    if n == 0:
        return 1
    K = 1
    while n > 1:
        if n % 2 != 0:
            K *= x
        x *= x
        n //= 2
    return K * x

def fact(n): 
    arr = {}
    temp = n
    for i in range(2,int(n**0.5)+1):
        if temp % i == 0:
            cnt = 0
            while temp % i == 0:
                cnt += 1
                temp //= i
            arr[i] = cnt
    if temp != 1:
        arr[temp] = 1
    if arr == {}:
        arr[n] = 1
    return arr
    
def main():
    N = int(stdin.readline().rstrip())
    A = list(map(int,[int(x) for x in stdin.readline().rstrip().split()]))
    ans = 0
    
    for i in range(1,N):
        if A[i] < A[i-1]:
            ans += abs(A[i-1]-A[i])
            A[i] = A[i-1]
        
    print(ans)
            
main()