#include <bits/stdc++.h>
using namespace std;
using P = pair<int, int>;
using ll = long long;
int main() {
  int n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < (n); i++) cin >> a[i];
  int ans = 0;
  for (int i = 1; i < n; i++) {
    if (a[i - 1] > a[i]) {
      ans += a[i - 1] - a[i];
      a[i] = a[i - 1];
    }
  }
  cout << ans << endl;
}
