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