#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 stools[n];
  for (int i = 0; i < n; i++) {
    stools[i] = 0;
  }
  for (int i = 1; i < n; i++) {
    if (arr[i] < arr[i - 1]) {
      int diff = arr[i - 1] - arr[i];
      arr[i] += diff;
      stools[i] = diff;
    }
  }
  int sum = 0;
  for (int i = 0; i < n; i++) {
    sum += stools[i];
  }
  cout << sum;
}
