#include <bits/stdc++.h>
using namespace std;
int n, a[222222], ans;
int main() {
  scanf("%d", &n);
  for (int i = 0; i < n; i++) scanf("%d", &a[i]);
  for (int i = 1; i < n; i++) {
    if (a[i] > a[i - 1]) continue;
    ans += a[i - 1] - a[i];
    a[i] = a[i - 1];
  }
  printf("%d", ans);
  return 0;
}
