#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];
  }
  int stool = 0;
  for (int i = 1; i < n; i++) {
    if (arr[i] >= arr[i - 1])
      continue;
    else {
      stool += (arr[i - 1] - arr[i]);
      arr[i] += (arr[i - 1] - arr[i]);
    }
  }
  cout << stool << endl;
}
