#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;
using ll = long long;
using ld = long double;

#define all(x)  x.begin(),x.end()
#define v(x) vector<x>
#define nl '\n'
#define fxd(x) fixed << setprecision(x)
template<class t> using ordered_set = tree<t, null_type, less<t>, rb_tree_tag, tree_order_statistics_node_update>;
template<class t> using ordered_multiset = tree<t, null_type, less_equal<t>, rb_tree_tag, tree_order_statistics_node_update>;


int main()
{
    ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);
    int n; cin >> n;
    v(ll) arr(n);
    ll cnt =0;
    for(int i = 0; i < n ; i++) cin >> arr[i];
    for (int i = 1; i < n; i++)
    {
        if(arr[i] < arr[i-1])
        {
            cnt += arr[i-1]-arr[i];
            arr[i] = arr[i-1];
        }
    }
    cout << cnt;
}