#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int a[n];
  for (int i = 0; i < n; i++) cin >> a[i];
  int m = a[0];
  long long sum = 0;
  for (int i = 0; i + 1 < n; i++) {
    m = max(m, a[i + 1]);
    if (a[i] > a[i + 1]) {
      sum = sum + m - a[i + 1];
    }
  }
  cout << sum << endl;
}
