#include <bits/stdc++.h>
using namespace std;
void solve() {
  int n, ans;
  ans = 0;
  cin >> n;
  int arr[n];
  for (int i = 0; i < n; i++) {
    cin >> arr[i];
  }
  for (int i = 1; i < n; i++) {
    if (arr[i - 1] > arr[i]) {
      ans += arr[i - 1] - arr[i];
      arr[i] += arr[i - 1] - arr[i];
    }
  }
  cout << ans;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  solve();
  return 0;
}
