#include <bits/stdc++.h>
using namespace std;
using Int = long long;
const char newl = '\n';
template <typename T1, typename T2>
inline void chmin(T1 &a, T2 b) {
  if (a > b) a = b;
}
template <typename T1, typename T2>
inline void chmax(T1 &a, T2 b) {
  if (a < b) a = b;
}
template <typename T>
void drop(const T &x) {
  cout << x << endl;
  exit(0);
}
template <typename T = int>
vector<T> read(size_t n) {
  vector<T> ts(n);
  for (size_t i = 0; i < n; i++) cin >> ts[i];
  return ts;
}
signed main() {
  cin.tie(0);
  ios::sync_with_stdio(0);
  int n;
  cin >> n;
  auto as = read(n);
  int ans = 0, pre = 0;
  for (int a : as) {
    chmax(pre, a);
    ans += pre - a;
  }
  cout << ans << newl;
  return 0;
}
