#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 < (int)(n); i++) cin >> a[i];
  int m = 0;
  int ans = 0;
  for (int i = 0; i < (int)(n); i++) {
    if (a[i] < m) {
      ans += m - a[i];
    }
    m = max(m, a[i]);
  }
  cout << ans << endl;
  return 0;
}
