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