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