#include <bits/stdc++.h>
using namespace std;
void solve() {
  int n;
  cin >> n;
  int a[n];
  for (auto i = 0; i < n; i++) cin >> a[i];
  int ma = a[0];
  int cnt = 0;
  for (int i = 1; i < n; i++) {
    if (a[i] < ma) {
      cnt += ma - a[i];
    } else
      ma = a[i];
  }
  cout << cnt << "\n";
}
int main() {
  solve();
  return 0;
}
