#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 max_num = 0;
  int sum = 0;
  for (int i = 0; i < N; i++) {
    if (A.at(i) >= max_num) {
      max_num = max(A.at(i), max_num);
    } else {
      sum += max_num - A.at(i);
    }
  }
  cout << sum << endl;
}
