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