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