#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int arr[n];
  for (int i = 0; i < n; i++) cin >> arr[i];
  if (n == 1) {
    cout << "0" << endl;
    return 0;
  }
  int cnt = 0;
  int maxi = arr[0];
  for (int i = 1; i < n; i++) {
    if (arr[i] < maxi) {
      cnt += (maxi - arr[i]);
    } else {
      maxi = arr[i];
    }
  }
  cout << cnt << endl;
  return 0;
}
