#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int arr[n];
  for (int i = 0; i < n; i++) {
    cin >> arr[i];
  }
  int sum = 0;
  for (int i = 0; i < (n - 1); i++) {
    if (arr[i] > arr[i + 1]) {
      sum += abs(arr[i] - arr[i + 1]);
      arr[i + 1] += abs(arr[i] - arr[i + 1]);
    }
  }
  cout << sum << endl;
  return 0;
}
