#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int ans = 0, temp;
  int *arr = new int[n];
  for (int i = 0; i < n; i++) {
    cin >> arr[i];
  }
  for (int i = 0; i < n - 1; i++) {
    if (arr[i] > arr[i + 1]) {
      int diff = arr[i] - arr[i + 1];
      ans += diff;
      arr[i + 1] += diff;
    }
  }
  cout << ans << endl;
  delete[] arr;
  return 0;
}
