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