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