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 sim template < class c
#define ris return * this
#define dor > debug & operator <<
#define eni(x) sim > typename \
enable_if<sizeof dud<c>(0) x 1, debug&>::type operator<<(c i) {
sim > struct rge { c b, e; };
sim > rge<c> range(c i, c j) { return rge<c>{i, j}; }
sim > auto dud(c* x) -> decltype(cerr << *x, 0);
sim > char dud(...);
struct debug {
#ifdef LOCAL
~debug() { cerr << endl; }
eni(!=) cerr << boolalpha << i; ris; }
eni(==) ris << range(begin(i), end(i)); }
sim, class b dor(pair < b, c > d) {
ris << "(" << d.first << ", " << d.second << ")";
}
sim dor(rge<c> d) {
*this << "[";
for (auto it = d.b; it != d.e; ++it)
*this << ", " + 2 * (it == d.b) << *it;
ris << "]";
}
#else
sim dor(const c&) { ris; }
#endif
};
#define imie(...) " [" << #__VA_ARGS__ ": " << (__VA_ARGS__) << "] "
#define cool ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define int  long long int
#define pb push_back
#define fe first
#define lb lower_bound 
#define ub upper_bound
#define pii pair<pair<int,int>,pair<int,int> >
#define se second
#define endl "\n"
#define pi pair<int, int>
#define mi          map<int,int>
#define mii         map<pi,int>
#define vi vector<int> 
#define vvi vector<vi>
#define bs   binary_search
#define rep(i,a,b) for(int i=a;i<b;i++)
#define rep1(i,a,b) for(int i=a;i<=b;i++)	
#define all(c) (c).begin(),(c).end()
#define sz(x)       (int)x.size() 
#define PI 3.14159265358979323846

 const int N=1e6+5;

 int mod= 1e9+7;
 int dx[4]={0,0,+1,-1};
 int dy[4]={+1,-1,0,0};  
 
 
int po(int a, int b) {
	a %= mod;
	int res = 1;
	while (b > 0) {
		if (b & 1)
			res = (res * a) % mod;
		a =(a*a) % mod;
		b >>= 1;
	}
	 return res;	
  }
   

int n;
	
void solve() {
  cin>>n;
  vi a(n,0);
  rep(i,0,n) {
	      cin>>a[i];
	  }
  int lt=a[0],sm=0;	  
  rep(i,1,n) {
	    if(a[i]<lt) {
			   sm+=abs(a[i]-lt);
			}
		else 
		   lt=a[i];	
	  }	 
	  
	  cout<<sm<<endl;   
    
                   
}

    
int32_t main() {
	cool;
	int t=1;	

	//cin>>t;
	while(t--)
	  solve();

  return 0;	   
}