//  Increasing Array

#include<bits/stdc++.h>

using namespace std;
typedef long long ll;

int main() {
    int n;
    cin >> n;
    vector<ll> array(n);
    for (auto &e : array) cin >> e;
    ll count = 0;
    for (int i = 1; i < n; i++) {
        if (array[i] < array[i-1]) {
            count += array[i-1] - array[i];
            array[i] = array[i-1];
        }
    }
    cout << count << endl;
    return 0;
}