#include <bits/stdc++.h>
using namespace std;
const int mod = (int)1e9 + 7;
int main() {
  int n;
  cin >> n;
  int a[n];
  for (int i = (0); i < (n); i++) cin >> a[i];
  int mx = a[0], ans = 0;
  for (int i = (1); i < (n); i++) {
    if (a[i] < mx)
      ans += mx - a[i];
    else
      mx = max(mx, a[i]);
  }
  cout << ans << endl;
  return 0;
}
