#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 maxi = a[0], res = 0;
  for (int i = 1; i < n; i++) {
    if (a[i] < maxi)
      res += maxi - a[i];
    else
      maxi = a[i];
  }
  cout << res << endl;
  return 0;
}
