#include <bits/stdc++.h>
#define fast ios_base::sync_with_stdio(0); cin.tie(0)
#define int int64_t
#define ii pair<int, int>
#define endl '\n'
#define vi vector<int>
#define vii vector<ii>
#define vvii vector<vii>
#define F first
#define S second
#define pb push_back
#define FOR(i,l,r) for(int i=l; i<r; ++i)
#define ROF(i,r,l) for(int i=r; i>l; --i)
#define all(x) x.begin(), x.end()
#define amin(a,b) a = min(a,b)
#define amax(a,b) a = max(a,b)
#define test int t; cin >> t; while(t--) solve();

using namespace std;

typedef long long ll;

const int MAXN = 2e5+73;
const int MAXINT = 2147483098;
const int MAXLL = 9223372036854775258LL;
const int MOD = 998244353;




void solve(){
    int n;
    cin >> n;
    vi a(n);
    FOR(i,0,n) cin >> a[i];
    int ans = 0;
    FOR(i,1,n){
    int temp = max(0LL, a[i-1]-a[i]);
    ans+=temp;
    a[i]+=temp;
    }
    cout << ans;

}





int32_t main()
{
    fast;
    //test;
    solve();
    //freopen("kotlin.in", "r", stdin);
    //freopen("kotlin.out","w", stdout);



    return 0;
}
