#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  cin >> N;
  vector<int> A(N);
  for (int i = 0; i < N; i++) {
    cin >> A.at(i);
  }
  int ans = 0;
  int kijun = A.at(0);
  for (int i = 1; i < N; i++) {
    if (A.at(i) < kijun) {
      ans += (kijun - A.at(i));
    }
    kijun = max(kijun, A.at(i));
  }
  cout << ans << endl;
}
