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