#include <bits/stdc++.h>
using namespace std;
int main() {
  int N;
  cin >> N;
  std::vector<int> A(N);
  for (int i = 0; i < N; i++) {
    cin >> A[i];
  }
  int stepSum = 0;
  for (int i = 1; i < N; i++) {
    if (A[i] >= A[i - 1]) continue;
    int diff = A[i - 1] - A[i];
    stepSum += diff;
    A[i] += diff;
  }
  cout << stepSum << endl;
  return 0;
}
