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