#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[i];
  }
  int ans = 0;
  for (int i = 1; i < N; i++) {
    if (A[i - 1] > A[i]) {
      ans += (A[i - 1] - A[i]);
      A[i] += (A[i - 1] - A[i]);
    }
  }
  cout << ans << endl;
  return 0;
}
