#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int res = 0;
  vector<int> a(n);
  for (int i = 0; i < n; ++i) cin >> a[i];
  for (int i = 1; i < n; ++i) {
    res += max(a[i] - a[i - 1], 0);
    a[i] = max(a[i], a[i - 1]);
  }
  cout << res;
}
