#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < n; i++) cin >> a[i];
  int ans = 0;
  for (int i = 0; i < n - 1; i++) {
    int more = 0;
    if (a[i] > a[i + 1]) {
      more = abs(a[i] - a[i + 1]);
      a[i + 1] += more;
      ans += more;
    }
  }
  cout << ans << "\n";
  return 0;
}
