#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, stool = 0;
  int temp;
  cin >> n;
  long a[n];
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  for (int i = 0; i < n - 1; i++) {
    if (a[i + 1] < a[i]) {
      temp = a[i] - a[i + 1];
      stool = stool + temp;
      a[i + 1] = a[i];
    }
  }
  cout << stool << "\n";
}
