#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;
  for (int i = 1; i < N; i++) {
    if (A.at(i) >= A.at(i - 1)) continue;
    ans += (A.at(i - 1) - A.at(i));
    A.at(i) = A.at(i - 1);
  }
  cout << ans << endl;
  return 0;
}
