#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 max = a[0];
  int s = 0;
  for (int i = 1; i < n; i++) {
    if (a[i] > max) max = a[i];
    if (a[i] < max) {
      s += max - a[i];
      a[i] = max;
    }
  }
  cout << s << endl;
}
