#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main(void) {
  int n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  int front_max = a[0];
  int ans = 0;
  for (int i = 0; i < n; i++) {
    if (a[i] > front_max) {
      front_max = a[i];
    } else if (a[i] < front_max) {
      ans += front_max - a[i];
    }
  }
  cout << ans << endl;
  return 0;
}
