Problem:
Score : 300 points

Problem Statement

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.

Constraints

1 \leq N \leq 2\times 10^5

1 \leq A_i \leq 10^9

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N
A_1 \ldots A_N

Output

Print the minimum total height of the stools needed to meet the goal.

Sample Input 1

5
2 1 5 4 3

Sample Output 1

4

If the persons stand on stools of heights 0, 1, 0, 1, and 2, respectively, their heights will be 2, 2, 5, 5, and 5, satisfying the condition.

We cannot meet the goal with a smaller total height of the stools.

Sample Input 2

5
3 3 3 3 3

Sample Output 2

0

Giving a stool of height 0 to everyone will work.

Solution in C++ (GCC 9.2.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using namespace std;
typedef unsigned long long ull;
typedef long long ll;
#define ll long long
#define ld long double
#define MOD 1000000007
#define nl cout << bl
#define rt ll Q[100005] = {0}
#define mp make_pair
#define test() \
    ull t;     \
    cin >> t;  \
    while (t--)
#define pb push_back
#define ff first
#define ss second
#define all(v) v.begin(), v.end()
#define s(ar, n) sort(ar, ar + n)
#define rs(ar, n) sort(ar, ar + n, greater<ll>())
#define oa(a, n)               \
    for (ll i = 0; i < n; i++) \
        cout << a[i] << " ";
#define cn(a, n)               \
    for (ll i = 0; i < n; i++) \
        cin >> a[i];
#define ov(a, n)                    \
    for (ll i = n - 1; i >= 0; i--) \
        cout << a[i] << " ";        \
    nl
#define ovp(a, n)              \
    for (ll i = 0; i < n; i++) \
        cout << a[i].ff << " " << a[i].ss << bl;
#define maxa(ar, N) *max_element(ar, ar + N)
#define mina(ar, N) *min_element(ar, ar + N)
#define present(c, x) ((c).find(x) != (c).end())
#define fastio()                  \
    ios_base::sync_with_stdio(0); \
    cin.tie(0);                   \
    cout.tie(0)
typedef pair<int, int> pii;
ll fast_exp(ll base, ll exp)
{
    ll res = 1;
    while (exp > 0)
    {
        if (exp % 2 == 1)
            res = (res * base) % MOD;
        base = (base * base) % MOD;
        exp /= 2;
    }
    return res % MOD;
}
ll fib(ll n)
{
    if (n < 2)
        return n;
    ll a = 0, b = 1, pre;
    int i = 1;
    while (i < n)
    {
        pre = (a + b) % MOD;
        a = b;
        b = pre;
        i++;
    }
    return pre;
}
ll fac(ll n)
{
    if (n % 2 == 0)
        return 2;

    for (ll i = 3; i * i <= n; i += 2)
    {
        if (n % i == 0)
            return i;
    }

    return n;
}
ll fact(ll n)
{
    if (n > 1)
        return n * fact(n - 1);
    else
        return 1;
}
bool sortbysec(const tuple<int, int, int> &a,
               const tuple<int, int, int> &b)
{
    return (get<1>(a) < get<1>(b));
}
int last_digit(int n)
{
    int ans = 0;
    while (n != 0)
    {
        ans = n % 10;
        n /= 10;
    }
    return ans;
}
int main()
{
    int n;
    cin >> n;
    int a[n];
    cn(a, n);
    ll res = 0;
    for (int i = 1; i < n; i++)
    {
        if (a[i] < a[i - 1])
        {
            res += abs(a[i] - a[i - 1]);
            a[i] = a[i - 1];
        }
    }
    cout << res;
}