#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
  int n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < (n); i++) cin >> a[i];
  int ans = 0, cnt = a[0];
  for (int i = 1; i < n; i++) {
    if (cnt > a[i])
      ans += cnt - a[i];
    else
      cnt = a[i];
  }
  cout << ans << endl;
}
