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