#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.precision(10);
  int n;
  cin >> n;
  int arr[n];
  for (int i = 0; i <= n - 1; i++) {
    cin >> arr[i];
  }
  int ans = 0;
  for (int i = 1; i <= n - 1; i++) {
    if (arr[i] < arr[i - 1]) {
      ans = ans + arr[i - 1] - arr[i];
      arr[i] = arr[i - 1];
    }
  }
  cout << ans << "\n";
  return 0;
}
