#include <bits/stdc++.h>
using namespace std;
template <typename P, typename Q>
P CeilDiv(P _dividend, Q _divider) {
  return static_cast<P>((_dividend - 1LL + _divider) / _divider);
}
template <typename Tp>
inline void outarr(Tp _begin, Tp _end, const char* _delim = " ") {
  for (Tp current = _begin; current != _end; ++current) {
    std::cout << *current << _delim;
  }
  std::cout << '\n';
}
using ll = long long;
using pii = std::pair<int, int>;
constexpr int INF = 0x3f3f3f3f;
constexpr int MOD = static_cast<const int>(1e9 + 7);
int main() {
  std::ios_base::sync_with_stdio(false);
  std::cin.tie(nullptr);
  int n;
  cin >> n;
  vector<int> arr(n);
  for (int i = 0; i < (n); ++i) {
    cin >> arr[i];
  }
  ll ans = 0;
  for (int i = 1; i < n; ++i) {
    if (arr[i] < arr[i - 1]) {
      ans += arr[i - 1] - arr[i];
    }
  }
  cout << ans << "\n";
  return 0;
}
