#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  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]) {
      int diff = a[i - 1] - a[i];
      ans += diff;
      a[i] += diff;
    }
  }
  cout << ans << "\n";
  return 0;
}
