#include <bits/stdc++.h>
using namespace std;
int main() {
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  int n;
  cin >> n;
  vector<int> a(n + 1);
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
  int sum = 0;
  for (int i = 2; i <= n; i++) {
    if (a[i] < a[i - 1]) {
      sum += a[i - 1] - a[i];
      a[i] = a[i - 1];
    }
  }
  cout << sum << "\n";
  return 0;
}
