#include <bits/stdc++.h>
using namespace std;
const long long mod = 1000000007;
int main() {
  int n;
  cin >> n;
  int a[n];
  for (int i = 0; i < n; i++) cin >> a[i];
  int ans = 0;
  for (int i = 0; i < n - 1; i++) {
    if (a[i] > a[i + 1]) {
      int diff = 0;
      diff = a[i] - a[i + 1];
      a[i + 1] += diff;
      ans += diff;
    }
  }
  cout << ans << endl;
  return 0;
}
