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