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