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