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;
#define mod (int)1000000007
#define MOD (int)10000000007
#define X 1001100011100001111ll
#define M 26388279066623ll
#define all(a) a.begin(),a.end()
#define for0(i, n) for (int i = 0; i < n; i++)
#define for0e(i, n) for (int i = 0; i <= n; i++)
#define for1(i, n) for (int i = 1; i <= n; i++)
#define loop(i,a,b) for (int i = a; i < b; i++)
#define bloop(i,a,b) for (int i = a ; i>=b;i--)
#define tc(t) int t; cin >> t; while (t--)
#define ll long long
#define pb emplace_back
#define fio ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(NULL)
#define in(x) scanf("%d", &x)
#define rr return 0
#define prec(n) fixed<<setprecision(n)
#define maxpq priority_queue<int>
#define minpq priority_queue<int, vector<int>, greater<int> >
#define inf (int)(1e18)
#define ini(a, i) memset(a, i, sizeof(a))
#define vi vector<int> 
#define F first
#define S second
#define pi pair<int, int>
#define ppi pair<pair<int,int> , int >
#define vii vector<pi>
const int MAXN = (int)((1e5) + 100);
int max(int a,int b){if(a>b) return a; else return b;}
int min(int a,int b){if(a<b) return a; else return b;} 
int gcd(int a,int b) { if (a == 0) return b; return gcd(b%a, a);}
int cbrt(int x){ int lo=1,hi=min(2000000ll,x);while(hi-lo>1){int mid=(lo+hi)/2;if(mid*mid*mid<x){lo=mid;}else hi=mid;}if(hi*hi*hi<=x) return hi;else return lo;} 
const int dx[4] = {-1, 1, 0, 0};
const int dy[4] = {0, 0, -1, 1};
const int N =  (int)(1e6+5); 
// ========================================================
signed main(){
  #ifndef ONLINE_JUDGE
  //freopen("input.txt", "r", stdin);
  //freopen("output.txt", "w", stdout);
  #endif
  fio;
    int n;cin>>n;
    vector<int> a(n);
    vector<int> stool(n);
    int mx = INT_MIN;
    for0(i,n){
        cin>>a[i];
        mx = max(mx,a[i]);
        stool[i] = mx;
    }
    ll ans = 0;
    for(int i=0;i<n;i++){
        if(a[i] < stool[i]){
            ans += stool[i] - a[i];
        }
    }
    cout<<ans<<endl;
  rr;
}