#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  int count = 0;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < n; ++i) {
    cin >> a[i];
  }
  for (int i = 1; i <= n; ++i) {
    if (a[i] < a[i - 1]) count += a[i - 1] - a[i];
  }
  cout << count << endl;
  return 0;
}
