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