#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, arr[200005], total = 0;
  cin >> n;
  for (int i = 0; i < n; ++i) {
    cin >> arr[i];
  }
  for (int i = 1; i < n; ++i) {
    if (arr[i] < arr[i - 1]) {
      total += abs(arr[i] - arr[i - 1]);
      arr[i] += abs(arr[i] - arr[i - 1]);
    }
  }
  cout << total << endl;
  return 0;
}
