#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 count = 0;
  for (int i = 0; i < n - 1; i++) {
    if (arr[i + 1] < arr[i]) {
      count += arr[i] - arr[i + 1];
      arr[i + 1] += arr[i] - arr[i + 1];
    }
  }
  cout << count << endl;
  return 0;
}
