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