#include <bits/stdc++.h>
using namespace std;
int main() {
  int N, count, countmae;
  cin >> N;
  count = 0;
  vector<int> vec(N);
  for (int i = 0; i < N; ++i) cin >> vec[i];
  for (int j = 0; j < N - 1; ++j) {
    if (vec.at(j) > vec.at(j + 1)) {
      countmae = count;
      count = (vec.at(j) - vec.at(j + 1)) + countmae;
      vec.at(j + 1) = vec.at(j);
    }
  }
  cout << count << endl;
}
