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