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