
// Problem : C - Step
// Contest : AtCoder - AtCoder Beginner Contest 176
// URL : https://atcoder.jp/contests/abc176/tasks/abc176_c
// Memory Limit : 1024 MB
// Time Limit : 2000 ms
// Powered by CP Editor (https://github.com/cpeditor/cpeditor)

/*ヽ｀、ヽ｀｀、ヽ｀ヽ｀、、ヽ ｀ヽ 、ヽ｀🌙｀ヽヽ｀ヽ、ヽ｀
ヽ｀、ヽ｀｀、ヽ 、｀｀、 ｀、ヽ｀ 、｀ ヽ｀ヽ、ヽ ｀、ヽ｀｀、
ヽ、｀｀、｀、ヽ｀｀、 、ヽヽ｀、｀、、ヽヽ、｀｀、 、 ヽ｀、
ヽ｀｀、 ヽ｀ヽ｀、、ヽ ｀ヽ 、 🚶ヽ｀｀｀｀ヽヽヽ｀、、ヽ｀、、ヽ*/
#include <bits/stdc++.h> 
#pragma GCC optimize ("O3")
#pragma GCC target ("sse4")
 
#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
 
 
using namespace std;
using namespace __gnu_pbds;
 
typedef long long   ll;
typedef long double ld;
typedef complex< ld > cd;
 
typedef pair<int, int> pi;
typedef pair<ll, ll>   pl;
typedef pair<ld, ld>   pd;
 
typedef vector< int > vi;
typedef vector< ld >  vd;
typedef vector< ll >  vl;
typedef vector< pi >  vpi;
typedef vector< pl >  vpl;
typedef vector< cd >  vcd;
 
#define l00p(i, a, b) for (int i=a; i<(b); i++)
#define loop(i, a)    for (int i=0; i<(a); i++)
#define rep1(i, a, b) for (int i = (b)-1; i >= a; i--)
#define rep(i, a)     for (int i = (a)-1; i >= 0; i--)
#define trav(a, x)    for (auto& a : x)
#define uid(a, b)     uniform_int_distribution<int>(a, b)(rng)

#define sz(x) (int)(x).size()
#define mp make_pair
#define pb push_back
#define f  first
#define s  second
#define lb lower_bound
#define ub upper_bound
#define all(x) x.begin(), x.end()
#define ins insert
//#define MOD 998244353
 
template<class T> bool ckmin(T& a, const T& b) { return b < a ? a = b, 1 : 0; }
template<class T> bool ckmax(T& a, const T& b) { return a < b ? a = b, 1 : 0; }

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

const ld pii=3.14159265359; 
const int MOD = 1000000007;
const char nl = '\n';
const ll INF = 1e18;
const int MX = 100001; //check the limits, dummy

int main() {
	ios_base::sync_with_stdio(0); cin.tie(0); 
	int n;
	cin>>n;
	vi a(n);
	int ans=0;
	loop(i,n)
	cin>>a[i];
	int mx=a[0];
	l00p(i,1,n)
	{
		if(a[i]<=mx)
			ans+=mx-a[i];
		else
		mx=a[i];
	}
	cout<<ans<<nl;
	return 0;
}
// read the question correctly (ll vs int)
// template by bqi343