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