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