#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
  int n, ans = 0;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < (n); i++) cin >> a[i];
  int now = a[0];
  for (int i = 1; i < n; i++) {
    if (a[i] < now) {
      ans += now - a[i];
    } else {
      now = a[i];
    }
  }
  cout << ans;
}
