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