#include <bits/stdc++.h>
using namespace std;
int n, a[10000010], ans = 0;
int main() {
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) scanf("%d", &a[i]);
  for (int i = 1; i < n; i++)
    if (a[i] > a[i + 1]) {
      ans += a[i] - a[i + 1];
      a[i + 1] = a[i];
    }
  printf("%d\n", ans);
}
