#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int a[n + 5];
  for (int i = 0; i < n; ++i) {
    cin >> a[i];
  }
  int prev = a[0];
  int ans = 0;
  for (int i = 1; i < n; ++i) {
    if (a[i] < prev) ans += (prev - a[i]);
    prev = max(a[i], prev);
  }
  cout << ans << '\n';
  return 0;
}
